Basic implementation of CORS

Does not include: Credentials support, proper CORS cache support

I've written cors.rs to be (sort of) reusable. The CORSRequest and CORSResponse classes will probably
be extended to create FetchRequest and FetchResponse for the Fetch crate.

I've tried to do as much as possible in cors.rs, since the full CORS implementaion (and much of the code in the net crate) should eventually go to the Fetch crate.


Blocks #2282