Added new API wrappers. Updated to support JSON APIs. Added API wrappers for create Clients and Loans in Json API and getting Custom Fileds information API.

1. New API wrappers.
2. Added support for JSON content type for Mambu API requests. a)
Modified MambuApiServices, RequestExecutor , GsonUtils, APIData to
support additional  methods to specify the contentType in API requests,
specifying non-default date/time formats and generating API requests
with application/json content type.
b) Created LoanAccountExtended class ( in apisdk.model) to support json
object generation and parsing it to the format required by "Create
LoanAccount json API".

3.  Implemented API wrappers for creating Clients and Loans using json
format (ClientService and LoanService).

4. Implemented wrappers to support getting Custom Fields information
(can only be tested with Mambu 3.3).
5.  Added Centers API wrappers.