Updated to support Mambu 3.8 API changes

1. Added support for MBU-6661 (As a Developer, I need to update a
single custom field).

1.1. Added support for HTTP PATCH method in RequestExecutorImpl
1.2. Enhanced  ApiDefinition and ServiceExecutor classes to support
providing an optional “related entity” ID
1.3. Added wrapper support for new APIs to Update and Delete a single
Custom Field for any of the following entities:
Clients and Groups (ClientService.java), LoanAccounts (LoanService),
SavingsAccounts (SavingsService), Branches and Centres
(OrganizationService) and Users (UserService)
1.4. Added helper method in ServiceHelper
1.4 Updated corresponding DemoTest services. Updated DemoUtil to
provide helper methods for DemoTest services
1.5.  Updated Maven’s pom file to use version 4.2.1 for httpClient jar
(org.apache.httpcomponents).
Note: HTTP PATCH is available starting with the httpClient version 4.2

2. Added support for MBU-6620 and MBU-6621 (adding “Identifier” and
“AccountName” as new configurable fields for Transaction channels)

2.1  Created new methods for account transactions accepting Transaction
Channel fields. These new methods take TransactionDetails object as
input parameter. Specifically:
For LoanService:  disburseLoanAccount() and  makeLoanRepayment()
For SavingsService: makeWithdrawal() and makeDeposit()
2.2. The original implementations of these methods are marked as
@Deprecated and could be removed in future releases
2.3 Updated ServiceHelper to provide helper methods
2.4 Updated DemoTest services accordingly. Added helper methods in
DemoUtil.

Misc:
- Marked as @Deprecated wrapper methods relying on default pagination
parameters when a corresponding method with pagination parameters is
available:
getClients(boolean), getRapaymentsDueFromTo(),getLoanAccountRepayments()
- Refactored RequestExecutorImpl to use helper methods to process HTTP
response (common to most HTTP methods)
- Updated README file

These enhancements are available in the 3.8 version of the build
(Mambu-APIs-Java-3.8-bin.jar)
Updated with 3.8 pull request review comments

1. Corrected double underscore in ApiType.DELETE__OWNED_ENTITY
2. Removed unneeded entityId parameter in
makeParamsForUpdateCustomField() (ServiceHelper)
3. Reimplemented makeParamsForUpdateCustomField() making JSON string
using Gson.toJson() for CustomFieldValue object (instead of building
the JSON string by string concatenation.)
Merge pull request #36 from mdanilkis/master

Updated to support Mambu 3.8 API changes