Added end to end message latency calculations

This commit adds new statistics about how long individual messages take
from being published to being finished.  We maintain several percentile
values, the exactly quantiles which can be specified as runtime
parameters to nsqd.  In addition, this information is displayed quite
shnazily in nsqadmin.
Merge pull request #280 from mynameisfiber/master

nsqd: track stats of end-to-end message processing time