Roundrobin (doc update)

Updates to README and command line docs.  I hadn't submitted this before because I wasn't sure these interface changes would be merged.
No, this is now required in the server too.  The server uses it to populate the header that enables sticky routing.

I think we can trivially make it optional in the server if you want; it's just removing the last check in

https://github.com/getlantern/flashlight/blob/master/flashlight.go#L49

then an empty `Host` will be passed to enproxy and it won't populate the sticky host header.  I didn't do this by default because it seemed like unnecessary divergence.