Objm2 - initial refactor of the object model

This branch contains the intial refactor of the object model. It works with the example event server in the java-bandwidth-examples repo. Hightlights include:

1. Subclasses for all events
2. Factory methods on resource objects (Call, Bridge, Event)
3. Singleton client
4. Vistor pattern for event callbacks
5. Creds passed by env vars
6. Optional env var for endpoint and version
7. Introduction of AppPlatformException to wrap exceptions
8. Elimination of private scope for easy subclassing
9. Convenience methods for creating/making calls, speakSentence, playAudio, createGather and createBridge 

Be gentle! My day job is product owner.