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 MW method it links to is also called HangupAll, and since this is ofono backend, let's just leave it like that; it is much more readable that way