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 can change the spec. Initially it was on the TelephonyCall, but since there is no ConferenceCall object, and it only applied to conf calls, made sense to expose it on TelephonyManager