New feature: a "vendor" in "assets"

You now can define a "vendor" dir in the "assets" dir to store external
packages assets - the original "assets" containing jQuery and plugins
had been renamed "vendor_assets".

A new abstract class to rule the package architecture (the
Assets\Loader and the Assets\Package\Cluster extends it).
NOTE - You now need to define the RELATIVE path for assets (2nd
argument of the Assets\Loader class) ; relative from the project root.

Upgrade to version 0.0.7