Created by: josephearl
Initial CancellationToken & CancellationTokenSource implementation to match the C# API (system.threading.cancellationtoken and system.threading.cancellationtokensource) and the cancellation API on Android (https://github.com/BoltsFramework/Bolts-Android/pull/53 and https://github.com/BoltsFramework/Bolts-Android/pull/56).