Preflight header support.

[cluke009]

I had some issues getting this to work with angular.js and discovered it was the lack of preflight headers.

I added -p as an argument to the http server. So far this is working for me but I am pretty new to python so let me know if there are any issues.

[kchodorow]

Looks pretty good! I made a couple of minor notes, mostly formatting, and had a question about do_OPTIONS, which I haven't seen before.