Changed the UserManagerInterface and GameManagerInterface to be like
singletons.
Whoops, meant to put the singleton pattern on the CLASS, not the
INTERFACE.
More fiddling around with the UserManager and GameManager design. Got
rid of UsernameAndPassword object.
Modified to match the new UML diagram. Added comments. Implemented
singleton pattern.
Added comments and method signatures for the game manager classes too,
while I was there.