Added support for CORS

RFC @doomspork.

This is for issue #20

Need some input on this one. This is tested and works. I went back and forth on how and where to implement this. Let me know if you have any ideas where else this snippet should sit. Also I should be testing the disabled state of CORS. I am not sure how to test the disabled state, It would require a new instance of the app, or to unload the middleware. In any case, need your input :smile: