Adding in API to allow for DVR information used to drive a player UI

Added new class called videoElementExt with and API to help with DVR window. We could add other video element API to this as well in the future. Added metric to hold the information for DVR streams. Wanted the API to be a singleton and the metric VO to just hold the values for the API to use.
The player uses 'extension' to describe objects that the html page can replace to extend dash.js
In this case you might have a DVRWindow object with time, duration, seek, etc.. and have it use a DVRWindowExtensions for any functionality that should be extendable.