Elements now store their responsive rules, fixes for mobile-backgrounds

Previously, the Responsive class tried to return an array of the Rules that were applied to an element.  It makes more sense to have those Rules installed directly into the Element.  Upon installation, the Element activates the rule and applies the Rule's class to itself.

Also modified Element to lazily instantiate its classes array and style hash.  Element also gets a test class.

Also fixed some bugs related to mobile overrides on background colors and images.  There is more leniency in attribute names - mobile-background can be used instead of mobile-background-image, mobile-bgcolor and mobile-background-color are interchangeable.