Detach MediaSource correctly when no longer required
Created by: davemevans
Given the discussion at https://github.com/w3c/media-source/issues/53 and the linked W3C spec text Best practices for authors using media elements, we should probably be following that advice in order to release correctly any resources which are no longer needed.
I have tested this on Safari (9.0.3), IE11, Edge, FF (Nightly) and Chrome.