[EME] Key Release

https://www.w3.org/Bugs/Public/show_bug.cgi?id=17199

The Key Release portion (
http://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html#key-release)
of the proposal hasn't received a lot of feedback, so I'd like to start a
discussion about it.

Section 4.1 gives a good overview of the problem. Briefly, the goal is the
provide the application with secure proof that a key is no longer present
on the client ("released"). The application must also be able to ACK
proofs. One particular thing to note is that proofs are not related to any
particular media element. In addition, the current API proposal does not
associate key release with HTMLMediaElement or any other object.

Some possible topics for discussion:
 * Multiple KeyReleaseManagers could be created, but they would all
represent the same data. How might we make KeyReleaseManager global or a
singleton?
 * While not related to HTMLMediaElement from an API point of view, key
release would need to be tightly integrated with the implementation
underlying the rest of the proposal, which is related to HTMLMediaElement.
   - What is the impact on implementations?
   - How might we more closely associate key release with HTMLMediaElement
and/or the rest of the EME implementation?
 * How might representing sessions as objects (
https://www.w3.org/Bugs/Public/show_bug.cgi?id=16613) affect the design?