Creates constructors for all Models.Response.

Resolves https://github.com/octokit/octokit.net/issues/677.

Removes obscolete properties (gravatar).
Makes Models.Response properties all be protected (most were already).

Octokit/Models/Response/SshKey.cs

@@ -22,6 +22,14 @@ namespace Octokit
             Title = title;
         }
 
+        protected SshKey(int id, string key, string title, string url)
+        {
+            Id = id;
+            Key = key;
+            Title = title;
+            Url = url;
+        }
+
         /// <summary>
         /// The system-wide unique Id for this user.
         /// </summary>