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.
The ```style``` method is defined in ```form_screen.rb```. It retrieves, caches, and allow easily composing of styles defined in the hash that the ```styles``` method returns.