Fix/Rewrite Proxy

[jaffee]

The current Pilosa proxy has a couple limitations.

1. It only translates /query requests, and doesn't forward other requests to Pilosa.
2. It translates rows but not columns.
3. The API is a little weird to setup and use it.

We should probably rewrite it using something like https://golang.org/pkg/net/http/httputil/#ReverseProxy to handle most of the proxying details, and we can add in column translation on the response as well.