Remove global test helpers.
Merge pull request #579 from abuiles/remove-qunit-global-helpers

Remove global test helpers.
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.