Issue #58, WP action "slickquiz_save_quiz" and Javascript callback - $.fn.setPreSaveQuiz(function)

Hi @jewlofthelotus,

I've tidied up the Javascript callback mechanism.

And, I've tried to keep that and the new WordPress action as generic as possible - I'm sure there are other use cases out there, as well as the [one that I have cited](https://github.com/jewlofthelotus/SlickQuiz-WordPress/issues/58#issuecomment-40472752).

Many thanks,

Nick