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
Well, I can see the logic in that. Initially these methods were on TelephonyCall, but I had to move them to the backend because I was not allowed to refer to data members without getters; so TelephonyCall became a data class. This is an eclatant example how coding style requirements can screw up the logic of the code.