Feature/cdap 868 app fabric http handler refactor

First stab at refactoring ``AppFabricHttpHandler``. Moved all ``application`` level actions to class ``AppHelper`` and all ``program`` level actions to class ``ProgramHelper``.

Currently tested on standalone mode. Brought up CDAP, loaded multiple apps.  started services/flows. ran queries. No errors encountered. 

https://builds.cask.co/browse/CDAP-DUT290 passed.
This class is not temporary. It abstracts out all program lifecycle management operations. All HttpHandlers can call methods in this class for performing those actions. It achieves two purposes -
1. Takes most logic out of AppFabricHttpHandler, which delegates calls to this class. This way, we could have multiple versions of HttpHandlers use it.
2. The logic from this class could be moved up into AbstractAppFabricHttpHandler, but then that class will become huge. This way (with having AppHelper, ProgramHelper), we can achieve logical  splitting of business logic.


I did not split the HttpHandlers themselves because some methods in AppHelper need to call methods in ProgramHelper, so we would have had to have an instance of ProgramHttpHandler in AppHttpHandler.