styling helper that supports composite styles and caching

Summarized in the 'Styling' section in the README.md file. I'm pushing this out for review... it would need tests, and improved documentation, but I wanted to see if this is helpful to others.
I did just push a commit with this as a mixin, but am thinking this might be best to keep as a subclass. The reason is that if the user doesn't use this subclass, it won't be compiled in by RubyMotion at all. If we do it as a mixin, it'll be there always (I think), even if the user is not using it at all.