Simpler in-page invocation of dash.js players

 * Searches the DOM for all instances of the class 'dashjs-player'. It
then looks for those which have a source element defined with a type
matching 'application/dash+xml'.
 * A new MediaPlayer is instantiated for each matching videoElement and
the appropriate source is assigned.
 * The autoplay property of the videoElement is preserved. Any preload
attribute is ignored. This method should be called after the page
onLoad event is dispatched.