Added support for via header

This adds [via header](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.45) support to resolve #4. I thought it was safest to make this opt-in to avoid breaking any existing implementations where the via header may cause an issue for whatever reason - e.g. I noticed that you had removed the host header because of a dotcloud issue and I wanted to avoid any similar problems. It might be more 'correct' to make it opt out, as [the W3C 'via' spec](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.45) says that this header is mandatory for gateways and proxies. Let me know if you want it opt-out instead and I can change it.