adding stub for estimateCost for IndexRangeNode.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/ExecutionNode.h
fixed bugs useIndexRange.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/Optimizer.cpp