Added tests for issues #1 #2

drawingAHorizontalLineOverAnAlreadyDrawnLineShouldThrow() handles the issue of the illegal operation of drawing over a line that has already been drawn.

callingboxCompleteOnACompleteBoxShouldReturnTrue() tests the correct state of boxes being complete when completely surrounded by lines.

callingboxCompleteOnAnIncompleteBoxShouldReturnFalse() tests the correct state of boxes not being complete until they are completely surrounded by lines. This will progressively surround a box, testing at each step to ensure that the box is not complete until all 4 lines are drawn around the box.