Added classes for the cursor (represented as an _ ), player and tower.

The player class has credits to build towers and a health attribute which would depend on whether enemies finish the path through the board.

The tower superclass has attributes for its position, its damage per second, range and health. There are three subclasses for types of towers (represented by ▲, ✸, ◎) which each have different attributes and prices.

The cursor moves in each cardinal direction and will not move outside the walls or write over any towers. It has the same interface as the path direction (0 for North - 3 for West) and 6, 7 and 8 to place towers.