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
the whole telephony is legacy and old fashioned; hangup belongs to that vocabulary; introducing new names is confusing; let's just leave it at that :)