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.
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.
Added map matching to common Match protocl
Merge pull request #378 from aphyr/match-maps

Added map matching to common Match protocl
streams/pipe
Merge branch 'master' of github.com:aphyr/riemann
streams/rate: slight docstring clarification
Fix a couple bugs in nbhm-index expiry

Events with :ttl keys present but nil were not receiving default ttls,
and deletion of expired events didn't use delete-exactly, which could
lead to a race condition where expiry deleted newer versions of events.
Document the use of sets as predicates to (where...)
Merge pull request #382 from ulises/master

Document the use of sets as predicates to (where...)
Deprecate streams/over and streams/under
Merge branch 'master' of github.com:aphyr/riemann
Improved docstrings for with and default.
Deprecate within and without
streams/rate: document expiry better
JSON-decoded events now have correct timestamps.

Millseconds are not seconds.