improve documentation of Crafty.scenes (and minor code edits)

Added more detailed examples and explanations to the public
documentation. Also added internal code documentation explaining
_current and _scenes. My minor code edits should not affect any
behavior: I added missing semicolons, and set _scenes to inherit from
Object instead of Array, in accordance with how it's used. (Therefore it
makes the code intention clearer, a good thing for both humans and
javascript engines.)