Add bower clean

Added a script that parses each bower component's `bower.json` file in order to remove all files that are *not* listed as main files.

There are so many extra libs that bower installs that are not needed if we're just using the source file(s) to be included in the asset pipeline.

This was written as an alternative solution to [#58](https://github.com/42dev/bower-rails/issues/58), and ended up cleaning up our `bower_components` directories as an added bonus.