Refactory for Reference files + Gruntfile update

I had to make a refactory in the docs folder, in order to get only 2 files (reference.js and main.css) for the YuiDoc theme. 

The reference is as usual in: /docs/reference
The YuiDoc theme: /docs/yuidoc-p5-theme
The sources for the theme: /docs/yuidoc-p5-theme-src

Gruntfile.js was also modified to make automatic builds of the Reference when modifications to the sources are detected (by using `grunt watch` as usual).

NOTE: an `npm install` should be done before testing, to install the new `sass` dependency.