TODO
====
1.  Spawner for creeps.  
    Ability to setup for levels and waves
2.  Upgrate Towers
3.  More creep variants
4.  Implement building costs
5.  Implement Selling towers
6.  Implement coloring for in and out gate
7.  Implement bonus (or multiplier) for killing creeps closer to out gate
8.  audio for shooting at creeps
9.  better audio for creep exploding
10. diagonal tower is too spaced-out -- better graphics?


BUGS
====
1.  Overlay outside the canvas area