Less installation

Hi guys!

I continue my CSS cleanup: this time I installed Less.
So I changed the AssetManager.
The main modification is that the CSS are always merged, because we can't directly use the Less files.

I also changed the CSS dependency of the Login, Installation & CoreUpdater plugin pages.
The Login plugin use the same CSS files that the dashboard to allow theming.
Installation & CoreUpdater will use a base.css file, that are a compilation of a part of the Less files. The purpose is to prevent theming & to have less maintenance.

Finaly, I initiate the plugin PleineLune: a dark theme for Piwik.
I don't know if I did it the right way, but it allows me to activate it.

Cheers,
Thomas.