Roundrobin

Changes to enable serving behind DNS round robin and other DNS setups involving a non 1-to-1 mapping from DNS names to IPs.  Proxies that want to take advantage of this change need to specify a Host field in the Proxy struct.  The client NewRequest is changed to take a (possibly "") `host` string parameter.  If provided, it must be used to create the new request.
Does it freeze indefinitely, or just hang for a while?