"synchronous" Property

The UNITY_EDITOR flag does not work as intended. It is set whenever code compiles in the Editor, so it cannot be used to differentiate between running in Play Mode (inside the Editor) and running in an Editor class.
I introduced a property "synchronous" in Request that should be set to true when using Request inside an Editor class such as a menu command. Default is "false" and causes Request to use the ThreadPool and ResponseCallbackDispatcher.