Stubbed out the API for pull requests.

Still missing pull request files, commit lists and merge checks.

Octokit/Models/Response/PullRequest.cs

@@ -4,6 +4,16 @@ namespace Octokit
 {
     public class PullRequest
     {
+        /// <summary>
+        /// The URL for this pull request.
+        /// </summary>
+        public Uri Url { get; set; }
+
+        /// <summary>
+        /// The pull request number.
+        /// </summary>
+        public int Number { get; set; }
+
         public Uri HtmlUrl { get; set; }
         public Uri DiffUrl { get; set; }
         public Uri PatchUrl { get; set; }