Add Gist API

This PR is to support https://github.com/github/hub/pull/601.

/cc @mislav
[`g.client`](https://github.com/octokit/go-octokit/blob/gist_api/octokit/gist.go#L56) is a gosawyer client which stores headers for the lifetime of the client. It'd be better to make this per request based. One you get an instance of our wrapper `Request` you could set it there. You prob. need to make [this method public](https://github.com/octokit/go-octokit/blob/3c9d2aeee0c508268c6b24a4008ac1e2917075fd/octokit/request.go#L8).