Expose readyState property on scope.

HTMLMediaElements have a `readyState` property that is described [here](http://www.w3.org/TR/html51/embedded-content-0.html#ready-states). Basically `readyState` indicates exactly how much data has been loaded: none(0), metadata(1), current data (2), future data (3), and enough data (4).

I am using angular-media-player to build the interface for a music player, and I need to be able to show the user when we are still trying to load a given track. Right now there is no way to do this.