User event coalescing 

This PR adds user event coalescing (fixes #8):

* Enable coalescence by default on Serf. Please check the timings. The timings aren't yet configurable but this is the same as the rest of Serf. Will add these in a future PR.

* Add a `-coalesce` flag to `serf event`. This defaults to true.

* `serf event` outputs some feedback now of what event is dispatched and whether it will be coalesced.

* Docs for all of this