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.
Could we add tests that ensure that this returns `true` for own properties, inherited properties, and accessor properties, including properties that contain `undefined`, and `false` for properties that have been deleted?