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

Conflicts:
	lib/pipeline/accumulators/FirstAccumulator.js
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_First
EAGLESIX-2651: First: fixed bug; updated tests
EAGLESIX-2651: First: del test fn names if ours
EAGLESIX-2651: First: minor fix to text in test
Merge pull request #16 from RiveraGroup/feature/mongo_2.6.5_accumulators_First

Feature/mongo 2.6.5 accumulators First