[patch] Order CSS rules by pseudoclass importance

According to CSS spec, CSS pseudo-classes like :link, :visited, :hover and :active must be ordered properly to take effect [1]. This patch orders CSS definitions generated by sprite-factory in this exact order. Rule without a pseudo class comes first. Please take a look at https://github.com/amberbit/sprite-factory/blob/pseudo_classes/test/images/reference/hover.css to see the result. Files with --active suffix would be sorted as the last ones in their group.

Let me know if you're ok with this change and if this patch can be improved in any way.

[1] http://www.w3.org/TR/CSS2/selector.html#dynamic-pseudo-classes