Make IndexRange rule look for multiple variables in FILTERs.

Still TODO: Pickup further variables when seeing FILTERs.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/OptimizerRules.cpp
Finish taking into account multiple FILTER for IndexRangeRule.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2