Undo command

[rachx]

so that I can easily correct any accidental mistakes in the last command

[burnflare]

My thought of implementing this was: We update abstract class Command to add an abstract method called reverseExecute(). All Command classes would have to implement this method which basically does the exact opposite of what the execute() command originally did. Then we can just maintain a stack of Commands and as the user calls 'undo', get the last command from the stack and call reverseExecute() on it.