Bug 835758 - Lots of keypad duplicated code between apps/communication/dialer and apps/system/emergency-call 

There is a lot of code duplicated in the `apps/communication/dialer/js/keypad.js` and `apps/system/emergency-call/js/keypad.js`. That means that every time that there is a non-specific change to the keypad, both files have to be changed, making it easy to forget and to introduce bugs.

Both keypads should share a base implementation and add their particular behaviors to that implementation.

Relevant Mailing list conversation: https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.gaia/ifN6qZ3tZ5w