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.

Comienza con la configuración de las pruebas de integración del commit aff0a13d884ff5dc09bccb6c1ce081a82213bfab y compárala con las pruebas unitarias existentes que utilizan un servidor HTTP local. Define el alcance necesario antes de continuar; el resultado final debería ser una suite de la API de GitHub en vivo que detecte problemas de compatibilidad, como cambios en el tipo de medio predeterminado, sin comprobar cada campo de respuesta.

Escrito por el modelo de indexación a partir del texto del issue.