EAGLESIX-2651: initial port of FirstAccumulator from v2.6.5
EAGLESIX-2651: initial port of LastAccumulator from v2.6.5
EAGLESIX-2651: initial port of SumAccumulator from v2.6.5
EAGLESIX-2651: initial port of MinMaxAccumulator 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_Last

Conflicts:
	lib/pipeline/accumulators/LastAccumulator.js
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_MinMax

Conflicts:
	lib/pipeline/accumulators/MinMaxAccumulator.js
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_Sum

Conflicts:
	lib/pipeline/accumulators/SumAccumulator.js
EAGLESIX-2651: initial port of PushAccumulator from v2.6.5
EAGLESIX-2651: make create method in Push Accumulator static
EAGLESIX-2651: initial port of Avg Accumulator from v2.6.5
EAGLESIX-2651: initial port of AddToSet Accumulator from v2.6.5
EAGLESIX-2651: add missing call to reset() in AddToSet
EAGLESIX-2651: fix up test cases for new code
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_AddToSet
EAGLESIX-2651: AddToSet: improve tests; add merge test
EAGLESIX-2651: AvgAccumulator: add tests from 2.6.5
EAGLESIX-2651: AddToSetAccumulator: switch tests to use process like other accumulator tests
Merge branch 'feature/mongo_2.6.5_accumulators' into feature/mongo_2.6.5_accumulators_First
EAGLESIX-2651: AddToSet: minor fix to ctor test