ShapeMatcher now uses a percentage difference threshold between the areas of the objects.  Also updated threshold values.
Pruner.prune_inactive now has an optional parameter to specify how many frames an object has to be inactive before it gets pruned.  Defaults to 1.
Potential tracking objects now have to be tracked for 15 frames (up from 5) before they are considered legitimate objects.
Moved frame number field up to MultistageTracker which passes it to the other trackers.  CamShiftTracker will now update objects' last tracked frame number.  Tried pruning "orphaned" moving objects but there must be a better way to handle them.