Allow aborting a .save in before_save

Sometimes you may be aware there's a problem with a record in before_save, but you still want to push ahead with the migration. You can't simply catch the exception as the error will disrupt the database transaction and prevent everything from being committed. This allows before_save hooks to return False. Otherwise achieved by setting <code>instance.save = lambda : NOne</code>.
Document the changes to before_save
Merge pull request #5 from aidanlister/patch-1

Allow aborting a .save in before_save