Copy request body in ProxyServer (kubecfg -proxy)

Fixes #1149 - kubecfg proxy "411 Length Required" error on POST/PUT.
@lavalamp actually, is [ProxyHandler](https://github.com/GoogleCloudPlatform/kubernetes/blob/71c6f8ee5c793a641dfe2d9bb8a98b7a8dba1863/pkg/apiserver/proxy.go#L73) the alternative? It looks like an overkill for `kubecfg -proxy`. There are so many pieces involved, which probably make sense for apiserver proxy but I'm not sure about kubecfg proxy.

Wouldn't something like [httputil.ReverseProxy](http://golang.org/pkg/net/http/httputil/#ReverseProxy) make more sense?