[STABLE] Large Update (Physics, Items, Enemies)

This update includes an entire overhaul of the physics engine, now using
proper AABB collisions tests. Relies on more impulse based mechanics.

Also included, a new item! The 1-up mushroom now can be utilized in
game. Just as well,  accompanying this item, there is a new tile type
INVISIBLEITEMBLOCK, which allows you to hide items in the levels. These
blocks are penetrable from all directions except when hit from the
bottom to activate their item to pop out.

Another new addition is a brand new enemy, the Koopa! There are two
types at the moment: regular and flying. Flying koopas turn into regular
koopas if stepped on. Currently Koopas do not turn to shells once
"killed". This will be added in a later update.

Other fixes include overhaul of input controls for player; these are
smoother, and also include a new animation of the "skid" effect while
overshooting a direction when quickly turning around.

More to come soon!