[WIP] 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.

Octokit.Reactive/Clients/IObservableGistsClient.cs

@@ -91,6 +91,24 @@ namespace Octokit.Reactive
         /// <param name="since">Only gists updated at or after this time are returned</param>
         IObservable<Gist> GetAllForUser(string user, DateTimeOffset since);
 
+        /// <summary>
+        /// List gist commits
+        /// </summary>
+        /// <remarks>
+        /// http://developer.github.com/v3/gists/#list-gists-commits
+        /// </remarks>
+        /// <param name="id">The id of the gist</param>
+        IObservable<GistHistory> GetCommits(string id);
+
+        /// <summary>
+        /// List gist forks
+        /// </summary>
+        /// <remarks>
+        /// http://developer.github.com/v3/gists/#list-gists-forks
+        /// </remarks>
+        /// <param name="id">The id of the gist</param>
+        IObservable<GistFork> GetForks(string id);
+
         /// <summary>
         /// Creates a new gist
         /// </summary>