Bug #58, New WP action "slickquiz_save_quiz" - PHP component

* As detailed in the bug/ issue report, a way to save extra data
* https://github.com/jewlofthelotus/SlickQuiz-WordPress/issues/58
Bug #58, Set a callback to add extra data sent with Ajax POST - JS

* Needs work!
* Javascript to complement the PHP modifications in previous commit
* New public function `$.fn.setPreSaveQuiz(function () { return extra })`
Bug #58, Tidy up the Javascript callback mechanism

* Public JS function `$.fn.setPreSaveQuiz(function ( return extra ) {})`
this git/svn setup is killing me