CSS borderimage support

Hi,

And the final patch of addings - mainly the 'border-image' support, along with insets usage. The truth is that I've coded it to use [UIColor colorWithPatternImage] which does not support resizable images, just for UIView this code won't be extremely useful, until Apple make this work, or we decide to add some UIView while applying the style, which I feel is not the idea of Nimbus.
One other thing is added: [NICSSRuleset pathPrefixForImages] it was needed for the purposes of main css to work - since URL of images in css are given relative to css's root (should they?).

Regards,
-Jonan