Move Crafty.scene into a new separate file

Crafty.scene should not be in drawing.js.  This commit moves it into a new file, "scene.js".
Extend Crafty.scene to allow passing parameters

This commit allows a scene to be called with an optional parameter, which is passed to the init function of the scene.

It also adds unit tests for both the new and original functionality of Crafty.scene.
Create .defineScene and .enterScene functions

This patch splits the functionality of scene into two functions: defineScene and enterScene.

It also adds tests for this functionality, as well as checking uninit.