Add Gist API

This PR is to support https://github.com/github/hub/pull/601.

/cc @mislav
Oh never mind, we don't need to make `newRequest` public. You could use [`client.NewRequest`](https://github.com/octokit/go-octokit/blob/3c9d2aeee0c508268c6b24a4008ac1e2917075fd/octokit/client.go#L29) instead