Fixes Issue #34.  Bug fix and test code addition for make_distance_based_randomizer

This commit adds test code and a bug fix for make_distance_based_randomizer.  Emphasizing the importance of test code, it looks like the old version only randomized the tips *around* the tip of interest (based on PhyloNode.getTipsWithinDistance() excluding the tip that calls it), rather than the tip + all tips within a distance being randomized.  The new behavior is to randomize all nodes within a given distance of the node of interest on the tree.   The result that used that code will need to either be updated to describe the procedure that was actually performed or (better) rerun for manuscript revisions.