Gists - get Commits and Forks

- [ ] Finish Gists API Implementation
  - [ ] [Add method to get gist
commits](https://developer.github.com/v3/gists/#list-gist-commits)
  - [ ] [Add method to get gist
forks](https://developer.github.com/v3/gists/#list-gist-forks)

Fixes #328, Fixes #216

Added implementation for the remaining pieces of the Gists API.  The others mentioned in #328 and #216 were completed through other PRs. Needs review, but I believe it is merge ready.