allows assets also to be found under app/Resources

currently assetic is not able to override bundles assets within the `app/Resources` Folder.

this patch enables the normal inheritance known from Twig-Templates. (http://symfony.com/doc/2.1/book/templating.html#overriding-bundle-templates)

1. `app/Resources/FooBundle/public/css/foo.css` will override
2. `src/Bundle/FooBundle/public/css/foo.css` will override (child bundle)
3. `vendor/foo/FooBundle/public/css/foo.css`