Change all the things.

* Remove old libraries/config.
* Refactor Manifests.
* Remove code related to building assets from the helper.
* Allow hierarchical manifests.
* Allow blessing (splitting) css.
* Update options.
* Cleanup readme.
* Improve handling of assets with missing extensions.
* Use exceptions on error (rather than exit).
* Allow checking less imports for changes (sloooooow).