Add integration tests

This is an ongoing work item to add integration tests for the entire library. Unlike the existing unit tests that spin up a local HTTP server that mimics certain behavior of the API, these integration tests will call against the live GitHub API. As such, they will likely take a lot longer to execute and will not be run near as often. I've set things up with aff0a13d884ff5dc09bccb6c1ce081a82213bfab and will continue working from there.

I'm not 100% settled yet on how extensive these tests need to be. For example, we don't need to test the value of every field in a response. The biggest goal is to have a test suite that can be run when things like the [default media type changes](https://developer.github.com/changes/2014-04-09-reminder-about-default-media-type-change/) to make sure the library is still working properly. I also have a separate test suite that I'm working on to identify fields in responses that we aren't mapping into our Go structs.