EAGLESIX-2651: initial port of Avg Accumulator from v2.6.5
EAGLESIX-2651: AvgAccumulator: add tests from 2.6.5
EAGLESIX-2651: Avg: del test names if ours
Merge pull request #14 from RiveraGroup/feature/mongo_2.6.5_accumulators_Avg

Feature/mongo 2.6.5 accumulators Avg