W3C Telephony API implementation.

Implements the latest W3C Telephony API specification found at http://telephony.sysapps.org/
Requires the oFono telephony middleware and the BlueZ Bluetooth middleware for working with paired phones.

BUG=XWALK-1419
getParticipants is a request, the getParticipantsDone is the callback. is 'Finished' a better name? ;)