extensions headers: multiple headers support

Hi!

I use your lib extensively, and looking forward to adapting Teepee someday. But being pragmatic, I have to live with things working now. Until recently I was satisfied with your http lib enough, but now that I need to parse cookies I stumbled upon major design flaw in your lib: no cookies support, nor multiple extension headers with the same name support.

Not that I'm complaining, you explicitly noted in README the lib is incomplete and unsupported, but as I need some ways to handle cookies, I decided to go with simplest way: add support for multiple custom header with the same name and parse cookies myself.

Hence the PR. Hope it'll help somebody out there. This is temporary measure, and I know it, but it's better than nothing.

Thanks!