Upgrade to version 2 of the API

Hi,

I'm using the govtrack gem for an upcoming project and need to use version 2 of their API (when you wrote this library they were using version 1). I made as few changes as possible to make the gem and the new API compatible, but v2 has some significant differences from v1 and that's reflected in the commits. I wanted to offer a pull request in case you wanted to use the updated API as well.

Overview of my changes:

* All API calls are made to v2 of GovTrack
* Got all tests to pass (many failed with the change)
* Made sure all examples in your README were still functioning
* Took out methods and work-arounds that relied on deprecated features (like which fields you could search on)

Let me know if you have any questions. I learned a lot from reading your library.