Merge pull request #1048 from bcwaldon/proxy-monster

proxy mode
Procfile: add a proxy

Woo! Now we have a cluster and a proxy and etcdctl works when running
the procfile by default!
Merge pull request #1053 from philips/proxy-in-procfile

Procfile: add a proxy
proxy: reimplement ReverseProxy code

The ReverseProxy code from the standard library doesn't actually
give us the control that we want. Pull it down and rip out what
we don't need, adding tests in the process.