Add CONNECT proxy support [Fixes #146]

[em-connectify](https://github.com/ConradIrwin/em-connectify) is essentially a copy-paste of [em-socksify](https://github.com/igrigorik/em-socksify) but for the CONNECT protocol that squid uses.

I've integrated it in the same way for now. If you'd prefer to merge the code from em-connectify into em-socksify, I'm happy to do that.