ImageDirectory filter

When I'm composer to manage assets (bootstrap, jquery, font-awesome), some images and font files are not located in the web/ folder.

The purpose of this filter is to move them to a dedicated folder in ``web/``.

To be complete, this PR must still:

* [ ] be documented
* [ ] be configured in AsseticBundle
* [ ] be tested

Some possible enhancements:

* store metadata to avoid copy of files already present
* download URL

Did you already thought of that? Am I going in the good direction?