DataSync tizen plugin implementation

SPEC=https://developer.tizen.org/dev-guide/2.2.1/org.tizen.web.device.apireference/tizen/datasync.html

BUG=XWALK-1061

----------------------------------------------------

Running TCT:
 - install rpms on device:
     - libwbxml2,
     - sync-agent,
     - oma-ds-agent (and start: systemctl start oma-ds-agent.service),
 - copy 'tct-datasync-tizen-tests.wgt' to device,
 - install and run application,

Current TCT datasync passrate: 63/63

--------------------------------------------------
what about return const &?