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.

I think, TLS is a better example: it could operate using a variety of crypto algorithms, but procedure of negotiating is completely hidden and high-level client doesn't care what algorithm is chosen.