Removed side-effect in Room creation. Modified tests to compensate.

The Room was adding an aribtrary number of towers when it was
created. Moved that out to a separate function that doesn't get
called in the constructor. Tests are much more consistent, now.
Removed some commented-out code related to last commit
Subclassed the zombies, like we should have all along
Put default path finding method in path class. Made zombie constructors more elegant.