Support Gradients

[mhemesath]

I haven't ran into this yet, but I think we may need to consider adding support to gradients. Given:

<linear-gradient> = linear-gradient(
[ 
        [ [top | bottom] || [left | right] ] 
    | 
    <angle>
,]? 
<color-stop>[, <color-stop>]+
);

I think we'll either have to flip left to right and transform the angle along the x-axis or invert the ordering of the colors. Before this makes it in, I think I'd like to get a comprehensive test suite in making sure all cases are getting converted correctly.