Make capture* callbacks fire after sending, more API cleanup/singletonization

[LewisJEllis]

Short version: addresses #213, moves the API to follow raven-js almost exactly, still maintains BC in almost all ways with old API

Details:

• captureException etc now fire their callbacks after sending to the server, as in Fix the API around capture* and handle callback correctly #213
• Moved everything to a Raven class instead of Client, following raven-js pattern/API
  • We now export a default Raven instance, leading to almost-identical usage to raven-js
  • Still have a Client wrapper for bc/multi-instance capabilities
• Does some other polishing/cleanups on the API changes
• Adds glue to maintain backwards compat with old API as much as possible
  • None of the changes to tests actually affect any existing public API usage patterns at all

The only public API usage affected by this should be the one described by @mattrobenolt in #213, and the realistic caveat to run into from it is that existing code looking at eventIds from capture* callbacks will now get null instead of a string.

/cc @benvinegar

[benvinegar]

Looks okay, but it's hard to see through the indentation.

My only ask is: a test that demonstrates that the callback behavior has changed to triggering after send (vs before send).

[LewisJEllis]

@benvinegar pushed a commit for that.

[benvinegar]

How does this test guarantee that done() is called after the process callback?

[LewisJEllis]

Good catch, had mixed up with a change made in one of the other PRs and this wasn't checking it correctly. Fixed.