Reflect

The Reflect global is, as far as I know, a part of the ES6 spec. I was a bit surprised not to find it in the es6-shim. Is there any reason as to why it isn't? If not, I'd consider giving it a go.

I'd like to use this PR to discuss Reflect implementation.
Similarly, could we add tests that ensure that a mutator property works, that an own property gets modified, and that an inherited property gets shadowed with a new own property?