Add StatsD/Datadog integration

This should be fairly straightforward, I'll add comments inline wherever needed.
This PR needs https://github.com/boltdb/bolt/pull/212

@benbjohnson 

/cc @charlescng @snormore 
I haven't figured out a good, low overhead way to grab the stats in a thread-safe manner, but this should be good enough. The issue is that the stats are updated as part of the tx.close, and there's no way to squeeze the stats emission in there and starting a new write tx for this seems like an overkill.