Implement JavaScript HTTP proxy

TorBirdy really needs a HTTP->SOCKS5 shim - who better to provide it than TorBirdy?

I propose that we use Moxie's incredible head start on the matter: https://github.com/moxie0/Convergence/blob/master/client/components/LocalProxy.js https://github.com/moxie0/Convergence/tree/master/client/chrome/content/workers https://github.com/moxie0/Convergence/tree/master/client/chrome/content/proxy

There are two ways to do it - one is async and the other isn't. I think we should first try the most straight forward approach of using what is already written; later we can improve it by making it async.