Rough draft of EME Concerns document.

As discussed in various forums, this is a version of the opinion document that tries to catalog our concerns for EME as a web-facing API, without suggesting solutions or focusing on specific technical choices or problems with the current mechanism. Feedback definitely welcome.

The difference I am trying to get at is between the API being designed to block such things, and that coming about as a natural result of the ecosystem, like certain videos only being encoded in certain forms. Does that make any sense? If so, suggestions on how to make it clearer would be welcome.