Hand rolling hit detection on aliens and shield INSTEAD of using collidable

* Alien radius now a readable attribute
* Added Alien#hit? and Shield#hit?
* Removed collidable from Shield
Realized a HUGE design flaw. Now using 4 bases and 1 turret.

* Thanks to the following sites:
  * http://www.youtube.com/watch?v=Xr7TXeNSlxg I realized that I had too many turrets.
  * V2 might have more turrets like this: http://www.missilecommand.com/arcade.php