Add TowerPlacer class to handle tower placement

This class will handle the logic related to placing towers.  Right now
it uses the push buttons to place the tower, which will need to use the
mouse eventually.
Add a class to check if a switch is set

This is a very basic controller to check if a certain switch is on or
off.
Add a class to check if a key is set

This is a basic controller to check if a key is down or up.  There is no
debounce yet, it just checks whether the bit is set or not.
Add a BitOperations class

This holds a function to check if a specific bit is set.