Multicast DNS improvements and port selection stability

• Change source port randomization to avoid overflow in the port range
  (#83 from @yomimono).

Improve mDNS support (#82 from Luke Dunstan):

• Add Dns.Probe to implement the unique name probing portion of mDNS.
• Expose the Dns.Name.Set construct.
• Added a functor Mdns_resolver_mirage.Chain that is intended to compose
  an mDNS resolver with a normal DNS resolver such that *.local is resolved
  via mDNS and everything else is done with DNS.
• Changed Dns.Query to not respond to queries for classes other than IN.
• Fixed mDNS legacy responses to use TTL <= 10 sec
• Fixed mDNS responses to use RD=0.

Critical Dns_server_unix.listen regression bugfix

0.15.3 (2015-07-30):

• Fix regression in 0.15.2 which prevented Dns_server_unix.listen from
  answering more than one query (#80 from @MagnusS)

Mirage bugfixes and Lwt improvements

0.15.2 (2015-07-04):

• Fix incorrect mirage dependency on tcpip
• Improve clarity and formatting of Lwt use
• Remove lwt camlp4 dependency
• Now requires lwt >2.4.7

Fix critical DNS resolver timeout bug causing unexpected exceptions

0.15.1 (2015-07-02):

• Fix critical DNS resolver timeout bug causing unexpected exceptions

Name module improvements

0.15.0 (2015-05-14):

• Name.domain_name has been renamed to Name.t and is now abstract
• Name.domain_name_to_string has been renamed to Name.to_string
• Name.string_to_domain_name has been deprecated for Name.of_string
• Name.parse_name has been renamed to Name.parse
• Name.marshal_name has been renamed to Name.marshal
• Name.hashcons_charstring has been renamed to Name.hashcons_string
• Name.hashcons_domainname has been renamed to Name.hashcons
• Name.canon2key has been renamed to Name.to_key
• Name.for_reverse has been replaced by Name.of_ipaddr
• Name.of_ipaddr accepts a Ipaddr.t and produces a name suitable for reverse DNS
• We now require >= ipaddr.2.6.0 to support Name.of_ipaddr
• uri 1.7.0+ is now required for its uri.services service registry
• Named service lookups are now supported in zone files
• Dig string serializations are now in Dns.Dig (#61 from Heidi Howard)

Fix namespace pollution, add composition functions, Async_kernel support

• Reduce namespace pollution in name.ml to avoid breaking with Cstruct 1.6.0+.
• Add a Dns_server.compose function to make it easier to build resolution pipelines (#58).
• Add a Dns_server_mirage functor (#55).
• Add Dns_resolver.resolve_pkt to support custom query packets (#49).
• Split out the experimental Async_resolver into a Async_kernel and Unix libraries.
  This introduces the dns.async-unix library.

Interface improvements for DNS packet manipulation

• Renamed Packet.QM to Packet.Q_Normal and QU to Q_mDNS_Unicast for
  clarity and added more detailed doc comments. Added constructor function
  Packet.make_question for convenience. (#41)
• Support io-page 1.3.0+ interface. (#40)

Improved multicast DNS support

• Add support for multicast DNS (RFC6762) in the trie. (#35 from Luke Dunstan)
  • mDNS doesn't use SOA nor delegation (RFC 6762 section 12), so some minor changes
    to Trie are required to handle this.
  • mDNS doesn't echo the questions in the response (RFC 6762 section 6), except
    in legacy mode, so a bool argument was added to Query.response_of_answer.
  • Query.answer still exists but now Query.answer_multiple is also available
    for answering multiple questions in one query to produce a single answer
    (RFC 6762 section 5.3). One caveat is that responses may exceed the maximum
    message length, but that is not really specific to mDNS. Also, in theory multiple
    questions might require multiple separate response messages in unusual cases,
    but that is complicated and the library does not deal with that yet.
  • Query.answer_multiple takes an optional function to allow the caller to control
    the cache-flush bit. This bit is only set for records that have been "confirmed
    as unique". Using a callback requires minimal changes here but puts the burden of
    maintaining uniqueness state elsewhere.
  • Query.answer_multiple takes an optional function to filter the answer, in order
    to support "known answer suppression" (RFC 6762 section 7.1). Again, using a callback
    requires minimal change to the core, but later on the mDNS-specific known answer
    suppression logic could move into the Query module if that turns out to be simpler.
  • A query for PTR returns additional records for SRV and TXT, to support efficient
    service discovery.
  • Trie.iter was added to support mDNS announcements.
• Switch to Bytes instead of String for eventual -safe-string support.
• Partially remove some error printing to stderr. (#36)

Unit tests were added for some of the changes above, including a test-only
dependency on pcap-format.

Multicast DNS parsing support

• Parse and marshal the mDNS unicast-response bit (#29).
• Add OUnit tests for Dns.Packet.parse using pcap files.
• Fix parsing of SRV records (#30).
• Use Bytes instead of String for mutable buffers.
• Switch to Base64 v2, which uses B64 as the toplevel module name
  to avoid linking conflicts with other community libraries.

Add Async resolver and reduce Io_page dependencies

• Do not depend in Io_page; instead Dns.Buf.create now accepts an
  optional alloc parameter to use a custom allocator such as Io_page.
• Add Async DNS resolver modules from @marklrh (#22).
• Add a Dns_resolver_mirage.Static for a static DNS interface.