Add ewma stream

This adds an exponential weighted moving average stream that takes into
account the time between events. It takes a half-life in seconds. Events
are weighted based on their :time value rather than by the time they
arrive.
async-queue: better docstring example
Merge branch 'master' of github.com:aphyr/riemann
Changelog for 0.2.5
0.2.5
0.2.6-SNAPSHOT
Merge pull request #346 from DanielleSucher/ewma_stream

Add ewma stream
Merge branch 'master' of github.com:aphyr/riemann
streams/ewma: document r
common/Match: you can now match nil. Applies to where, match, etc.