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
how do you remove a call? don't you stop/hang up/cancel it? or did sometone get removed from the call? I know its spec related but I just dont get it