Force child views to inherit none style

[fnxpt]

When we set nuiClass to none in a view all its children should inherit
the same class.
This "Fixes #113"

[tombenner]

There are times where a user may want to set a view's style class to none and still be able to apply styling to one of its descendants, so this approach is probably a little too severe. Excluding all of a view's descendants from NUI's styling is now possible by setting the property exclude-subviews in a style class's definition. This has just been added, so I haven't written documentation for it, but please see the "Usage" section of #65 for usage details.