Updates to allow testing in py2 or py3
Separated polymaze and polygrid into two (internal) packages before further development.
New PolyViz tests and implementation. Allows creation and application
of simple styles to edges and shapes. Breaks maze tests/implementation.
Update maze tests and implementation to use new system. External interface
for maze is unchanged (e.g. CLI and demo work exactly as before).
Closes #4; Closes #5; Closes #6; Closes #7