Unit Tests and Stream API Methods

I was looking through your Python API client and couldn't find the Stream endpoints; this pull request has some code to work with streams, with method names based on your ruby client, and docstrings based on the web API docs. I also wanted to write some unit tests for the new code, so there's also a new tests/ folder using nose as a test runner.

Hopefully this is useful -- let me know if there's anything I should change.