Add screen tracking + refactors

* Added Screen tracking so you can treat your Roku app like a Mobile app instead of a website.
* The API has not changed, but has been augmented with the new `UA_trackScreen()`.
* Modified event tracking very slightly to comply with Google's recommendations. (Category and Action are required; the other values are optional.)
* Moderate refactors

It would be good to refactor this at some point to make it more object-oriented, so that you can have multiple trackers instantiated at once if one wishes. But that can come later.