Add some parameterized superclasses?

[sraaphorst]

We could easily create a parameterized superclass of the various MazeGenerators instead of the three unrelated ones that there will be when I create GraphMazeGenerator.

This could be extended to various other things, like the maze types themselves, which would force and encapsulate some standard functions, like starting cell, ending cells, walls, etc.

[sraaphorst]

This was done as part of the PR for #66 - we now have AbstractMaze, which handles maze dimensions, starting cell ending cells, and finding dead ends, as well as an AbstractMazeGenerator, which handles dimensions.