Add required_methods for decorator

otherwise we can't issue OPTIONS requests to the server, see
first comment at http://flask.pocoo.org/snippets/56/

src/pylorax/api/crossdomain.py

@@ -59,5 +59,6 @@ def crossdomain(origin, methods=None, headers=None,
             return resp
 
         f.provide_automatic_options = False
+        f.required_methods = ['OPTIONS']
         return update_wrapper(wrapped_function, f)
     return decorator