Allow '0' as a DTMF tone.
Remove explicit false check for DTMF tones.
Merge pull request #105 from Bat-o-matic/master

Allow '0' as a DTMF tone.
SIP.Utils.Promise = global.Promise || require('promise');

package.json: browserify omits the 'promise' module from the build, so as to
avoid adding unnecessary code.

Gruntfile.js: jasmine config provides Promise shim to PhantomJS
WebRTC/MediaHandler: inline callbacksLast usage instead of using Utils
Move Promise code from Utils.js to Utils/Promise.js

It is imported by other modules as SIPPromise to avoid namespace collision

Utils.Promise still exists for use by tests
Utils/Promise: split addPromise and defer into own modules

This avoids overwriting properties of the global/imported Promise