New CSS properties

The first step in the larger effort, this set of changes adds the following properties to NimbusCSS:

top - the distance in pixels or percentage of the top of the target view from the top of the superview
bottom - the distance in pixels or percentage of the bottom of the target view from the bottom of the superview
left - the distance in pixels or percentage from the left (like top)
top - the distance in pixels or percentage from the right (like bottom)
width - pixels, percentage (relative to superview width), or auto for sizeToFit - negative pixels means superview width minus specified value
height - pixels, percentage, or auto for sizeToFit - negative pixels means superview height minus specified value
-ios-halign - center/left/right - aligns the view in the superview
-ios-valign - middle/top/bottom - aligns the view in the superview

This is my first pull to Nimbus, so don't be shy about what I screwed up and I won't take offense.