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.

> EME should not provide APIs that are designed to allow restriction of content to one key system
I'm not sure we will be able to force such a requirement. Content vendors won't trust unknown CDM.
Maybe it's better to formulate in terms "Content vendor may ask for a certain level of robustness and/or hardware support, but not concrete platform or CDM provider".