Adding the gist id back.

No "proper" synonym found that matches id, and we can't use id since
that's a Python builtin. So, just have a property called "gist".

    test_gist = Gist("abcdef")
    test_gist.gist == "abcdef"
    test_gist.url == "https://gist.github.com/abcdef"
Add support for GET with raw text body.

Not all requests come back as JSON.
Extra repository tests.

Testing the slug for extra edge cases, with malformed inputs.
Slug only exists when all information is provided.

Missing information now means that the slug doesn't exist.