Cors support (Addresses issue #367)

https://github.com/assaf/zombie/issues/367

Ok, this isn't quite working, but I would appreciate some input as I think it's close...

The only thing (i think) I'm having problems with is actually accessing the CORs header in xhr.coffee:

    unless current_host == res.headers['Access-Control-Allow-Origin']

(I accept this is fairly fundamental!)

Not sure if the problem is the test setup, or the way I'm trying to access it - I would really appreciate your thoughts.