Add screen tracking + refactors

[sfsekaran]

• 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.