Add lib and vendor to loose assets.

* Some gems locate loose assets in lib/assets or vendor/assets
* This change picks them up so they are processed during `rake assets:precompile`

Examples:

* https://github.com/thomas-mcdonald/bootstrap-sass/tree/master/vendor/assets/images
* https://github.com/joliss/jquery-ui-rails/tree/master/vendor/assets/images