CSS InterfaceBuilder support

Hi again,

This change concerns few small additions to make it possible to define styles for views directly from the InterfaceBuilder. Using "User defined runtime attributes" one can add 'cssStyles' string key and refer CSS-defined styles.
This very property is later defined in the UIView+UIStyleable category to handle xib loading calls. Since it needs to load a stylesheet from somewhere a new idea of "mainSheet" is introduced for the NIStylesheet and it is referred in application's Info.plist with the key 'NIMainCSSPath'.
So with those two one can use css styles for the interface without, actually writing any code.

I hope I've made those commits in a proper way so that you can easily add them.
Thanks,
-Jonan