Hopfactoring

Refactoring Rubyhop

* Create base Level class for the levels to inherit from
* Create MessageLevel for TitleLevel and FailLevel to inherit from
* Make RubyhopGame a singleton that is more easily accessed by sub classes
* Remove need to pass `@level` and `@window` around
* Some general cleanup
* Write some tests!

This should be a pure refactor -- no behavior changes.