Make into an ember-addon.

This PR turns `broccoli-asset-rev` into an Ember CLI addon.  It is using the API defined in https://github.com/stefanpenner/ember-cli/pull/1214 (which is actually pending a new version of `broccoli-asset-rev` that implements this).

That PR makes a `postprocessTree` hook available to all addons. This provides what is needed for `broccoli-asset-rev`, but also allows other addons to hook into the process (it even allows ordering `before` or `after` another addon).