v6.0.0

I made this a new major version because it includes a few changes that could in theory break something in some weird situations. But I think the vast majority of users (possibly all users?) won't have any issue upgrading.

• #48 - Switched to using DOMException errors rather than normal errors, since that's what the IndexedDB spec says to use, and Node.js now has a built-in DOMException in all supported versions.

• #93 - @bryan-codaio made the latest tweak to event scheduling, this time improving how setImmediate is used in some situations where people are mocking timers.

• #99 - @sjnho fixed handling of Date objects to account for some edge cases, including jsdom overriding the native Date constructor.