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/Application.cs

@@ -10,6 +10,14 @@ namespace Octokit
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class Application
     {
+        public Application() { }
+
+        public Application(string name, string url)
+        {
+            Name = name;
+            Url = url;
+        }
+
         /// <summary>
         /// <see cref="Application"/> Name.
         /// </summary>