Fix bug with circle renderers drawing circles off center.
extracted some values into constants. added first "zombie" but nothing happens
yet..
Implement basic bounding box collision. The player and the zombie can
collide. However, nothing happens (except a console.log)
Introduce collisions and vector2 modules as well as rectangle-circle
collision.

The collisions module handles collision detection for various shapes.
The vector2 module contains the Vector2 class.

Added "debug points" -- to debug why the circle rectangle collision
was behaving oddly. (I wasn't checking the bounds of the y or x values
after getting the intersection point between the edge of the rectangle
and the line from the center of the circle to the center of the
rectangle)

Added object names, purely for debugging purposes.
Remove drawing of debug points and console log statement.