typo--
Make every exercises use SimpleBuggles instead of Buggles.

- That way, Python that use the exercise entity instead of
  regenerating a new one will get a SimpleBuggle that behaves correctly
  (fix #121)
- We have to remove the "abstract" keyword of SimpleBuggle and add an
  empty run() method. This in turn forces to add the "override"
  keyword to all scala buggles defined in exercises. We should
  probably add a word about it in the first lesson where it's
  encountered.
- Note that the on disk worlds are also populated with SimpleBuggles.
  It makes it impossible to load worlds with Buggles (that raise
  exceptions). When we need it, we'll improve the file format.