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 you can just return a reference for a container.

IMHO this class is mainly carrying data, my idea is to use `struct`, so that all these spec names can stay readable. @halton what do u think?