Additional wrappers for Tasks, Documents and Organization services

1. RequestExecutor
- added support for DELETE method

2.  Tasks
- added getTasks() by clientId, username, task status
- added deleteTask() by id

3. Organization
- additional methods to get lists of Branches and Centres with full details (primarily needed to have addresses)

3. Documents (Demo only)
- added test to upload a document from the file
- added encoding helper method
- used test/data directory to place files for uploading (not checked in)

4.  DemoUtil 
- moved some good patterns for getting demo data (found in Tasks) into DemoUtil helper methods (getDemoUser() and getDemoClient())