Being able to overwrite a LESS file within a skin document

Currently, the LESS module only handle files on the FS, without looking at the current Skin object if a file has been overwritten.

It could be done by creating a temp directory in which we copy the overwritten files and that we pass to the LESS compiler as an "include path".

Since the LESS file are inside a folder inside the skin directory, we should also find a way to handle, in the skin object, the templates located in the subdirectory.