EAGLESIX-2690 expressions FixedArity
Merge branch 'feature/mongo_2.6.5_expressions_Nary' into feature/mongo_2.6.5_expressions_FixedArity

Conflicts:
	lib/pipeline/expressions/ToLowerExpression.js
	lib/pipeline/expressions/ToUpperExpression.js
EAGLESIX-2651: FixedArity: use NaryExpressionBaseT

* also minor change to error text
Merge pull request #20 from RiveraGroup/feature/mongo_2.6.5_expressions_FixedArity

feature/mongo 2.6.5 expressions FixedArity (into Nary branch)