add -prefix flag to add a prefix to all stat paths

I'm trying to run statsdaemon on every box in a cluster of machines, and need to add a prefix to every stat that includes the hostname (which then avoids duplicating the hostname getting and cleaning logic out of every application sending stats.)

This patch adds a `-prefix` option which allows you to add an arbitrary prefix to all stats sent to the daemon.

The whitespace and parentheses changes are from a `go fmt` run; they should be committed, but aren't part of this change.