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.
Merge pull request #955 from rjackson/fix-css-preprocesors

Provide EmberApp with registry access.