Expose Cascading pseudo-combiners

Add an AggregateByPump that is initialized either with existing Cascading
partial aggregators (e.g. SumBy, CountBy, etc.) or with a custom Functor
(map-side aggregator) and Aggregator. These must come after a grouping operation
and can be combined with each other. An aggregate by operation cannot be
combined with an every operation.
Merge branch 'master' of https://github.com/square/cascading-helpers

* 'master' of https://github.com/square/cascading-helpers:
  Bump version number
  Improve behavior of Pump.every with no arguments