changes to isometricrenderer

Added a namedtuple for increased readability
added more comments and docstrings.
correction of an inaccurate docstring in world.py
medium-sized chunkrenderer refactoring

Made the face types a bit flag.
Changed occlusion algorithm to match.
Changed compiled block definitions to include a FaceDef struct.
Factored out block rendering into its own function and exposed it to
python.
Changed the BlockDef struct to use packed bits instead of ints for the
flags.

Added comments throughout chunkrenderer.c