Add `app.import` to control legacy files, remove broccoli-bower.
better implementation of defaults in `EmberApp`

Lets loader + wrapInEval also be configurable.

We plan soon to make loader configurable via the underlying filter.
Please note, the current setup still does not allow this.
Use local trees instead of hardcoded folders.
Handle styles from `app.import`.
Allow populating only parts of `trees`.

Defaults only allows single depth merges.
Merge pull request #562 from rjackson/remove-broccoli-bower

Add `app.import` to control legacy files, remove broccoli-bower.