Added loose-end identification and updated the ASCII renderer for ChunkMaps.

* Added various methods for identifying loose ends in a ChunkMap.
* Changed the ChunkMap ASCII renderer to use two different types of block character and then alternate this like a checkerboard so different chunks can easily be identified.  It was SO confusing without this!
* Added a reverse_direction method to Chunk to make flipping directions like North to South easy.  Really this all needs to go in another class entirely, I'll refactor when I'm certain of what I need.