[EME] Spec now uses promises

I just committed a large
changeset<https://dvcs.w3.org/hg/html-media/rev/9842af174b80>that
implements promises per bug
25199.

The examples<https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/encrypted-media.html#examples>are
also updated*, and that is possibly the most interesting change.
MediaKeys and MediaKeySession creation is more complex, especially when
supporting more than one session in response to needkey events  as in example
8.2<https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/encrypted-media.html#examples-selecting-key-system>.
Since there may not be a MediaKeys object, initData must be queued. Also,
we need an additional variable to determine whether there is a pending
MediaKeys creation because video.mediaKeys is asynchronously set sometime
after MediaKeys.create() asynchronously completes.

There will be more changes soon as I resolve bugs that were dependent on
promises.

David

* Please let me know if there are errors or better solutions.