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
It seems `backend_` has same lifetime as TelephonyInstance, what about use object instead of pointer? then you don't have to new/delete for each TelephonyInstance's object.