Start using EventDispatcher for Services

[azasypkin]

Preliminary we want to import [1] and [2] including all existing unit tests. This will require some changes to make it more "es6-classy". And we need to decided whether we want it to be base class or mixin, or maybe both.

[1] https://github.com/mozilla-b2g/gaia/blob/master/shared/js/event_dispatcher.js
[2] https://github.com/vargin/event-dispatcher/blob/master/event-dispatcher.es6.js

[azasypkin]

It's being covered in #111 - MotionSensorService will be the first service that uses custom events, the rest of the services will expose events as needed.