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

Conflicts:
	lib/pipeline/accumulators/SumAccumulator.js
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_Sum
EAGLESIX-2651: Sum: update tests for 2.6.5; fix minor bugs
EAGLESIX-2651: Sum: use triple equality check
Merge pull request #21 from RiveraGroup/feature/mongo_2.6.5_accumulators_Sum

Feature/mongo 2.6.5 accumulators Sum