Scene parameters

This is based on #508, and allows a scene to be called with an optional parameter, which is then passed to the init function of the scene.

It also adds unit tests for both the new and original functionality of Crafty.scene.

I also moved `Crafty.scene` into a new file, `scene.js`.  I have no idea why it was in `drawing.js`; it has nothing to do with drawing...

thanks for the original PR, mikepaulson!