Add levelMixin to support multiple Level classes

Extract everything from Level into levelMixin
Change Level usage to be new Level[1]({ ... })
Call gameOver() if there is no next level (FIXME)
Add js/levels/123.js path for Level constructors
Add level 2, require defining goalsFulfilled

Every level class must define goalsFulfilled() method
so the game knows when the level is over.

Add level 2 which goalsFulfilled defined so that they never can be
fulfilled. (FIXME?)
Merge pull request #72 from scmx/multiple-level-classes

Add levelMixin to support multiple Level classes