Added ender.py to tracked files
Move sources to a subdirectory
Modify the main loop

src/game.py

    Make Game keep a stack of levels. The one on top is considered
    active.

    If the value returned by Level.step is None, the level is poped off
    the stack. If it's not the same Level, it's pushed on the stack.
    Nothing happens when it is the one already on top.

src/engine.py
src/sdl.py

    Make Engine.input return both an event and the delta time.

    Remove the Engine.dt property.
Sketch out boxes tests
Sketch out Box.__and__
Add Box.__contains__
Define tests for the set union operation

Conflicts:

	src/boxes.py
Add the method to test interection commutativity back
Move generation of test cases out into a base class
Add the tests for the intersection operation
Add the right-hand case of distributivity in the docstring
Add test case for the DeMorgan Laws
Add test case for union and intersection distributivity
Delete src/boxes_test.py
Clean up src/boxes.py
Delete src/collider.py
Add function to find the smallest bounding rectangle of two other rectangles
Add function to check if two rectangles collide
Fix and rephrase Box.__contains__
Add a bbfreeze script for building binaries -- tested on Linux