Support Pseudo-classes (e.g. Button:highlighted)

[tombenner]

This is currently handled with property suffixes (e.g. font-color-highlighted), but it should be done with pseudo-classes instead (e.g. Button:highlighted {font-color: black;}). We'll probably want to complete #33 before setting this up.

[benvium]

It's perhaps obvious, but It'd be good if you could set the background color in the highlighted state. With the current set of properties this isn't possible - you can change the text color and the background image, but not the background color.

EDIT: I just noticed that this isn't directly supported by UIKit. E.g. you can set an background image for a state but not a color. Anyone know a way around this?

[tombenner]

Regarding background-color-highlighted for Button, NUISettings has a getImageFromColor:withClass: method that could be used in conjunction with setBackgroundImage:forState:. I'll try to add this soon, but if anyone else wants to hit it first, feel free.