#43 – Proxied requests use the Upstream Host Header.

Hey @jehiah, here is a first take on an implementation of #43.

Could you provide feedback on the following:

1. This change breaks backwards compatibility in original Host header is no longer sent upstream. Would you like me to force the user to pass a flag (as in #1) to enable this new behaviour?
2. For the test, I was inspired by the [`reverseproxy` module's tests](http://golang.org/src/pkg/net/http/httputil/reverseproxy_test.go) to create a Host header echo server. It wasn't immediately clear which hostname I could use that would always to the local server... so I cheated and used xip.io. Any thoughts on how to not involve an this external dependancy?

Thanks!