v. 1.0 to 1.1
----------------------------

WHAT: Unknown cells are now seen as Free cells for base skeleton (before
pruning).  

REASON: In robotics, we use occupancy grids that classify the world as
Occupied, Free, and Unknown.  Using laser range finders, occupancy
grids often get lots of Free cells next to Unknown cells near the
frontiers.  v. 1.0 ignored Unknown cells (they could not "flip" during
the thinning process).  This resulted in lots of spurs near frontiers.
By treating Unknown cells as Free during the thinning stage, Unknown
cells are allowed to "flip", thus regions of Unknown cells are seen as
regions of Free space.  This gets rid of these spurs.

EFFECTS: Most users that only care about Occupied and Free cells will
notice no difference.  Users that do use all three bits of information
and run this a continuous loop for real-time applications, might
notice some slight slowdown.