Add addon.postprocessTree.

This hook allows any addon to hook into the final stage of the `toTree`
process. Making things like `broccoli-uglify` and `broccoli-asset-rev` able
to be addons and not need to live in the core of EmberCLI.

Also:

* Allows additional trees to be passed to `app.toTree`. Creating
  additional trees in the application `Brocfile.js` and merging them
  with the result of `toTree` is pretty common. Unfortunately, this
  means that any additional assets you have added will not be available
  for the post processing in addons. For example, any assets you add
  this way will never be fingerprinted.
* As the addon API develops it is odd that `included` and `treeFor` are
  required to be implemented since many plugins will never interact with
  the `EmberApp` stuff (for commands, preprocessors, middlewares,
  blueprints (soon), etc). This PR removes the errors from being thrown.