Added `$icon-` prefix to variable to resolve naming collisions

Add `$icon-` prefix to variable names to enhance compatibility with @groundworkcss and any other frameworks or projects that use simple, common variable names such as `$desktop`, `$tablet`, etc.

Hopefully you'll find this to be a simple, but valid enhancement to avoid naming collisions. I have included Font Awesome in https://github.com/groundworkcss/groundwork since alpha 0.1 and am I big supporter of your project

Keep up the great work!  :)