Added logic for compiling LESS and minifying CSS/JS

Using http://leafo.net/lessphp/ and YUI compressor. Please install YUI compressor using the command "brew install yuicompressor".

Hi Nicholas,

I'm using this for building media for my extensions. In combination with PHPStorm's File Watchers you can code LESS which is compiled and minified to file.min.css on-the-fly. Also the CSS directory is refreshed using a PHPStorm File Watcher. I didn't include the File Watcher, but this PR contains all logic for compiling/minifying.

If you like it I can edit the copyright headers. I suppose you'd like something like AkeebaBuilder or AkeebaBuildTools? Do you like this approach?

Cheers,
Jurian Even