Add unit tests

[gfairchild]

I've enabled Travis CI for this project. I need to implement unit tests. This shouldn't be hard (I'll more or less implement examples.py as unit tests), but the main barrier is that I can't actually query the real Yelp API because that would require a client_id and client_secret. I need to look up best practices when unit testing a client-side API implementation.

[gfairchild]

Looks like httmock may be useful here for mocking the behavior of Yelp.

[gfairchild]

Spent a few minutes looking more into this, and I don't think I'll need a mocking library after all. Travis CI contains support for defining sensitive variables as project-specific environment variables, which is exactly what I need. I can simply define my Yelp API credentials as environment variables, which will make unit tests a lot simpler to implement.

[gfairchild]

Closed by #12