Update CORS

ping @Ocramius @Orkin

I've updated the module. I've carefully read the Mozilla spec, and it seems that this is indeed better :): https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS

Especially check the example with the full exchange between client and server.

I didn't update test yet. But please Florent could you test this branch to see if it still work?