EAGLESIX-2651: initial port of Accumulator from v2.6.5
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_Accumulator

Conflicts:
	lib/pipeline/accumulators/Accumulator.js
EAGLESIX-2651: fix missing rhs function name
EAGLESIX-2651: Accumulator: fix misleading comment
Merge pull request #11 from RiveraGroup/feature/mongo_2.6.5_accumulators_Accumulator

Feature/mongo 2.6.5 accumulators Accumulator
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_AddToSet
EAGLESIX-2651: AddToSet: improve tests; add merge test