move renderers code into separate file. add dummy tests for game and
graphics engine (not yet implemented). some whitespace changes.
Refactored graphics engine slightly: removed listing of entry points
to GraphicsEngine methods in returned object from graphics/engine
module; instead just return the constructor for a GraphicsEngine.

Added a rendererstest which atm just tests the CircleRenderer's
draw function.

Added dummy tests for game.js and graphics/engine.js which don't
test anything yet.