serf: Adding new MemberReap event type
serf: Support coalesce of MemberReap
serf: Emit the MemberReap event
agent: Support filter on member-reap
website: Document the member-reap event
agent: Support the tombstone_timeout config
website: Document the tombstone timeout
Merge pull request #173 from hashicorp/f-reap

Adds the `member-reap` event.