Provide EmberApp with registry access.

* Allows customizing registry within `Brocfile.js`.
* Provides registry with `EmberApp` instance.
* Makes each `Plugin` have a `toTree` method. This will allow plugins that take a different signature to work properly (assuming they implement a custom `toTree`).
* Provides `name` property that is provided to `EmberApp` to plugin.  This allows the plugin to name the generated output file properly.

Fixes #954.
Fixes #827.