Add Gist API

This PR is to support https://github.com/github/hub/pull/601.

/cc @mislav
I asked [the same question](https://groups.google.com/forum/#!searchin/golang-nuts/RoundTripper|sort:relevance/golang-nuts/-j6p12SSpXI/aeF5mVwaeAcJ) on the mailing list :smile_cat:. The recommendation was to [make a shadow copy of the request](https://github.com/stretchr/gomniauth/blob/cd24d73410859a02f206ea8bec945bc5ea77a42e/oauth2/oauth2_tripper.go#L57-L67). [Another explanation](https://groups.google.com/forum/#!searchin/golang-nuts/RoundTripper|sort:relevance/golang-nuts/U4FPkQKcAzM/xPireKYNvowJ) on this topic. I looked at source a while ago and didn't find anything suspicious on the side affect of mutating request. But I could miss some info.