CDAP-538: Ported HelloWorld app to use Services instead of Procedures.
CDAP-538: Updated docs of Greeting service.
CDAP-538: Replaced sendJson to sendString and updated related tests.
CDAP-538: Removed TODO; Replaced service startup check to private method.
Merge remote-tracking branch 'upstream/develop' into CDAP-538-hello-world-app
CDAP-538: Used method from TestFrameworkTest to wait service startup.
Merge pull request #643 from cybervisiontech/CDAP-538-hello-world-app

CDAP-538: Ported HelloWorld app to use Services instead of Procedures.