Use relative position units

The sprite position is now defined as a percentage instead of a fixed pixel value.
Also the sprite icon itself is applied to the `:before` pusedo element and it's width and height defined in ems

This allows you to adjust the font size on your `.icon` class and the sprite icon will scale up accordingly.
Checkout the new demo page, I've added a slider that updates the font size and scales the icons.

What you think?