Merge pull request #1 from davidmason/master

add events to draw background and foreground
v0.1.0
remove sceneManager stuff from gameloop module
remove sceneManager stuff from gameloop module
cleanup
v0.2.0
add optional draw layers

If a property 'layers' is present and is an array, a set of 'draw-layer'
events will be emitted in addition to the usual draw event. This allows
entities to keep track of a z-index and draw in an appropriate plane,
without interfering with the current behaviour.