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.
Kirk, I talked to a few other player developers about this topic and the consensus is it must be unified.  It is not practical for every player developer using dash.js to have to first figure out what type of stream is being played and depending on the result go to different objects to query the values needed to populated and drive a custom control bar.  

Since we have access to the video element and we have knowledge of all the offsets being calculated for DVR and live playback, I think this very valuable and easy to achieve.    

I fully agree that we should remove the videoElementExt class and put the API in the MediaPlayer.js. This is an essential API for a player developer and is typically at this level for all other frameworks. 

I also think I should relocate some of the APIs that are Utils, such as the timeCode and UTC Date formatting (much more code for the date stuff still to come), and put them into a class that can be extended in case others want to format the values differently.