Pahfinding and Tower Manager

Fully implemented the pathfinding system.

Added a tower manager to handle all towers on the map
Enemy Movement

Added a test enemy and had it following the path created by the level
when the map is loaded.
Bugs:
Enemy can sometimes get stuck at a waypoint
-- Create a method to check if enemy is going to pass the waypoint and
start it moving to the next waypoint if true
Game crashes when enemy reaches end
--have enemy despawn and decrease life of the player when reaching the
end of the map
Testing Pathfinding

Tested pathfinding on longer map and tried tweaking the algorithm
slightly
Waves

Added waves to the game.  The WaveManager class loads the wave
information from a text file.  Every update it updates the current wave
and if it will spawn an enemy when the timer reaches 0.