Updated Centers and Branch APIs to retrun Lists (instead of an array)

Updated Centers and Branch APIs to return Lists (instead of an array)

Other changes include:
1. RequestExecutorImpl:  GET methods do not need ContentType
2.  Added new class: MambuApiResponseMessage : a helper to process exception
messages's content
3.  Added new class: APIExceptionData (in util).  Provides helper method to access
API response error codes by the responseStatus.  Documents API's
responseStatus constants.
4. MambiApiException: added new method to create Mambu Exception using
MambuApiResponseMessage  object.