Small update & Sass version of CSS

Hi there.  I thought that this was an interesting project and was hoping I could contribute.
If you're not interested in this commit, thanks anyway. It was fun to play around with.

Updates include:

• Move demo CSS into it's own file

• Update menu icon to be built from 1 span tag using :before & :after selectors to build the other two bars.  -- update demo.html, query.meanmenu.js & readme.md to reflect this change

• Replace <'s with &lt;'s on line 55 of readme, as they weren't rendering

• Add new scss file and compiled CSS and minified CSS files (slightly smaller than the original files)