trying to generalize scan, however drop is still missing...
found drop (it's called skip here...) - finished with generalizing scan
also adapted type signatures of the reduce methods
Also added aggregate (alias for reduce, see issue #20).
Fixed a typo, added missing error and completion handling
Switched away from using take and skip again due to concurrency issues.
Merge branch 'master' into improved-scan
fixed method signatures and respective javadocs of various aggregate/reduce/scan overloads
Merge pull request #257 from jmhofer/improved-scan

Improved scan, reduce, aggregate