Streaming

This has a long way to go. This pull request will allow for discussion as we work through it.

Quite a bit still to do:

- [ ] TESTS - proxy, request and response classes are lifted from EM-Twitter and virtually unchanged so those tests will be easily ported. Everything else currently lacks tests. I'm still uncertain about the structure of some things (particularly the relationship between the Client classes and the IO class)
- [ ] Reconnection handling (this got really messy in em-twitter: spagalloco/em-twitter#7)
- [ ] Error handling
- [ ] Stall handling - similar to reconnects, but this is a forced reconnect vs. reconnect on failure
- [ ] Response handling (i'd like to do something like intridea/tweetstream@2d270ef)
- [ ] Integration with Twitter gem credential configuration
- [ ] SSL Verification
- [ ] Ability to stop/restart
- [ ] TBD