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

Conflicts:
	lib/pipeline/accumulators/LastAccumulator.js
EAGLESIX-2651: Last: fix minor bug; update tests
EAGLESIX-2651: Last: tabs beats spaces
Merge pull request #17 from RiveraGroup/feature/mongo_2.6.5_accumulators_Last

Feature/mongo 2.6.5 accumulators Last