Optimized Menu Plugin

* Recoded to cache $(document) selector for performance
* Leverage JS assignment deconstructors to reduce duplication or code in menu parts access
* Cached distinct menu parts ( grouped selectors ) in the container node to reduce jQuery selector calls

Additionaly
* Added new ":discoverable" expression since ":focusable" and ":tabable" require the elements to visible. Due to CSS3 tranisitioning some elements in the menu were in not yet visible so expression was missing them.
  ":discoverable" extends ":focusable" except that it includes elements that are hidden ( or in the case of transitioning , becoming visible )