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/ApiError.cs

@@ -11,15 +11,20 @@ namespace Octokit
 #endif
     public class ApiError
     {
-        public ApiError()
-        {
-        }
+        public ApiError() { }
 
         public ApiError(string message)
         {
             Message = message;
         }
 
+        public ApiError(string message, string documentationUrl, IReadOnlyList<ApiErrorDetail> errors)
+        {
+            Message = message;
+            DocumentationUrl = documentationUrl;
+            Errors = errors;
+        }
+
         /// <summary>
         /// The error message
         /// </summary>