Created ArrayLookupNode::hasScalarKeys() and ArrayLookupNode::extractKeys().
ArrayLookupNode::getKey() now expects an index for which key to return (defaults to 0). Added a getKeys() method to return an array of cloned key nodes.
Wrote a test for getKeys().
Merge pull request #151 from grom358/extract_keys

ArrayLookupNode::hasScalarKeys() and extractKeys()