[WIP] 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.

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Still need to update `broccoli-asset-rev` to be an addon with this new API, and include it in the blueprinted apps `package.json`.
I intend to have 'all', 'js', and 'css', then move the minification all into addons.