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 
Not quite sure what you mean, that's what happens inside ddEmitStats(). I can grab the fresh stats here and pass them in as an argument if you prefer that. I had that originally here (with some additional guards which got simplified later) and decided to push it into ddEmitStats to not clutter Update too much. But I can go either way