CORS support

This approach is minimalist, it essentially provides a request helper that
extracts the CORS info, and an example of possible CORS logic.
This acknowledges that CORS logic can be custom and complicated, and leaves
maximum flexibility to the user.
Chrome and Safari send the Origin header on same origin header.

Origin and Host must be different for the request to be considered CORS request.
Merge pull request #43 from ant0ine/cors

CORS support