Adding physics interface to handle physics calls.

Define a general interface to handle physics and provide an interface
between the game and the physics engine.

Define functions in the interface to handle conversion from physics units
(meters) to game units (pixel).
Add world setup function.

Add function to setup the world to default settings defined by the
PhysicsInterface object. Function bounds the world by boxes to function
as walls/ground objects.
Add function to draw physics bodies.

Add function to iterate through and draw all physics bodies in the physics
world.

Fix dimension conversion functions in the Physics Interface.
Add box to physics world.

Test physics functionality by adding a box to the physics world.

(Note: Currently, the box loses its position on the first Step() call.
Under investigation.)
Fix physics problems by adding correct timestep.

Add custom implementation of gametimer average for possible use as the
timestep.

Fix timestep passed by engine to constant value determined by target
frame-rate (may need to look into developing a dynamic timestep based on
processing time to account for processing time if processing takes too
long).