AssetDirectory filter

When I use 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/``.

Exhaustive list of enhancements:

* A filter ``AssetDirectoryFilter`` copying all images to a directory in a public folder
* A new ``PathUtils`` class with common code between ``CssRewriteFilter`` and ``AssetDirectoryFilter``
* An object ``AssetDirectory`` to store images in a given folder