Add Gist API

This PR is to support https://github.com/github/hub/pull/601.

/cc @mislav
We could change the test fixture, but then we're not verifying in tests that the client is able to make cross-site requests. It's valuable information that requests hold the original "Host" header as we can then verify that they would be made to the correct host.

This is the reason for HUB_TEST_HOST in hub. Wondering if we could switch to something like that in this test suite. I wouldn't want it to be an environment variable, though.