Refactored View into separate classes, new MediaQuery class manages responsive styles

The View class was growing too long so I broke out the sub-classes into their own files - view/context and view/tag_stack.  There were no functional changes within those classes.

I've also added a new MediaQuery class which takes the array of Responsive::Rule objects formerly held by the View and manages it.  There were spots throughout the code, particularly in the tests, where having a first-class manager for the active rules would benefit and streamline code.