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
I think it makes sense to keep it: without the keyword, it's still possible to initialize `TelephonyCall` with an implicit conversion because `service` will default to `NULL`.