Improvements to the Physics engine.

Collision detection now works with the walls (had some walls outside of bounds in the y-axis).
Player shouldn't tunnel anymore through objects.
Added methods to create a larger assortment of World objects.

Note:  Collisions incorrectly detected on the top and left walls (too early), but not bottom and right.  This leads me to believe that the origin is simply incorrect in the Physics Engine.
Added an XnaGameDebugView so that we can see physics bodies and their interactions.  SpriteWithInputAndPhysics showcases this.
Physics now working as intended =)
Merge branch 'master' into inputphysics

Conflicts:
	src/demos/SpriteWithInputAndPhysics/SpriteWithInputAndPhysics/Program.cs
	src/demos/SpriteWithInputAndPhysics/SpriteWithInputAndPhysics/SpriteWithInputAndPhysics.csproj
	src/fwd.Xna/XnaGame.cs
	src/fwd.sln