Resizable Image Support

As described in my issue #44 I'm missing resizable image support.

In my opinion the best fitting design is adding a inset property for every mage.
As it fits the general design.

I  added an inset style able property for every image property available. Expect UIView background-image which is applied as color.

Example:
background-image
background-image-inset

I added an example in NUIStyle.nss:
background-image: NUIBarButtonTemplate.png;
background-image-insets: 5 5 5 5;