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
for 4-space indent for unfnished line, actually I prefer
```
notif["cmd"] = (key == "state") ?
               picojson::value("callStateChanged") :
               picojson::value("callChanged");
```