2.1.0-alpha1

• Changed: Copied most of the header-semantics for the PSR draft for
  representing HTTP messages. Reference here.
• This means that setHeaders() does not wipe out every existing header
  anymore.
• We also support multiple headers with the same name.
• Use Request::getHeaderAsArray() and Response::getHeaderAsArray() to
  get a hold off multiple headers with the same name.
• If you use getHeader(), and there's more than 1 header with that name, we
  concatenate all these with a comma.
• addHeader() will now preserve an existing header with that name, and add a
  second header with the same name.
• The message class should be a lot faster now for looking up headers. No more
  array traversal, because we maintain a tiny index.
• Added: URLUtil::resolve() to make resolving relative urls super easy.
• Switched to PSR-4.
• #12: Circumventing CURL's FOLLOW_LOCATION and doing it in PHP instead. This
  fixes compatibility issues with people that have open_basedir turned on.
• Added: Content negotiation now correctly support mime-type parameters such as
  charset.
• Changed: Util::negotiate() is now deprecated. Use
  Util::negotiateContentType() instead.
• #14: The client now only follows http and https urls.