core/reaper takes :keep-keys for expired events.

You can now pass {:keep-keys [:host :service :tags] to core/reaper, and
the reaper will preserve those fields in expired events. Useful if
you're trying to discriminate between expired events on the basis of
tags or descriptions. Closes #99; see ticket for discussion.
Merge branch 'master' of github.com:aphyr/riemann
streams/coalesce: proper use of transients

Lazily mapping back to persistent data structures allowed multiple
threads to use the transients. Closes #147
Small documentation fix for the function 'default'.
Merge pull request #151 from franckcuny/master

Very small documentation fix