Power User Mass Checking

Attached is a patch that adds the ability for power users to shift + click a checkbox to mass check all checkboxes above that checkbox.

A checkbox needs to be selected above the checkbox that is shift + clicked and all the checkboxes in the middle will be selected.

I also moved the code in ready() into its own function inside a Drupal.betterselect namespace. The namespace also has a check() function for handling setting the highlight class and if the check is checked. damn i said check wayyyy too much there...