Additional wrappers for Tasks, Documents and Organization services

1. RequestExecutorImpl
a) added support for DELETE method

2.  TasksService
a) added new wrapper getTasks() by clientId, username and/or task status
b) added new wrapper deleteTask() by id
c) updated createTask() to use JsonTask class
d) commented out unit test for createTask() for now (as it crashes after request on extraction form JsonTask)

3. OrganizationService
a)  Added additional helper methods to get lists of Branches and Centres with full details (primarily needed to get them with addresses)

3. DocumentsService 
a) Added new API wrapper getImage():  to download image files from Mambu (api/images)
In Documents Demo:
b) added  demo test to upload a document from a file
c) added file base64 encoding helper method for demo
d) added test/data directory to place files for uploading documents (with one test file)

4.  DemoUtil 
a) moved some good patterns for getting demo clients/users (found in Tasks) into DemoUtil helper methods (getDemoUser() and getDemoClient())