Less copying by returning a ref.
Improve comments for getVarusage and friends.
Fix bug with variable bounds: must not access document set in this node.
Make more const.
Make more const.
Fix more bugs in variable bounds.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/ExecutionBlock.cpp
	arangod/Aql/RangeInfo.cpp
	arangod/Aql/RangeInfo.h
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2