Start the NonLeafEntry with an empty bounding box so we don't get NPEs.
Fixes a bug where handleOverflow might not return a new parent node in adjustTree if the cooperating sibling isn't full.
Fixes a bug that arose from assuming all siblings are HRLeaf nodes when in fact, they can be HRNonLeafs too (any subclass of HRNode).
Merge branch 'master' of github.com:trxcllnt/hilbert-rtree