refactor lookupd utils for re-usability:

* and as a result refactor nsqadmin graph handling
nsqadmin: ensure that rates are never negative
nsqadmin: fix counter template error
nsq_stat: "iostat" like utility for aggregate NSQ cluster stats
Merge pull request #173 from mreiferson/nsq_stat_173

command line utility (nsq_stat)
nsq_to_http: add --host-pool option