Add utility mixins

[MattMcAdams]

Add utility mixins to allow advanced css short-hand.
Several ideas:

border($width, $style, $color, $radius);
margin($top, $right, $bottom, $left); // allowing null as a value
padding($top, $right, $bottom, $left); // allowing null as a value
position($position, $top, $right, $bottom, $left) // with new 'cover' position value
clearfix;
size($width, $height);
hidden($boolean); // hide visually, but keep access for screen readers