[CSS] Smal fixes, small css support added, nice feature added.

Hi, It's my first use of Nimbus and first contribution to it (and to open source projects at all). I've used only CSS part and fixed few small things. 
Extended a bit the CSS support - with 'border-top' and new '-mobile-image-XXX' stuff.
Probably, most important is the added ability to define css styles for a view, directly from within the InterfaceBuilder.

Hope my work will be useful for the others.
-Ivan Georgiev