Fix bugs in code

According to Dr Billingsley, there are two known bugs in our code-base:

Bug 1: The code for checking if a square is complete is wrong. It just returns true at the moment, whereas it should test if the lines surrounding the square have been "drawn".

Bug 2: The grid doesn't complain if you "draw" a line that has already been drawn. It should throw an IllegalStateException.

These bugs need to be fixed.

From Chantelle