Add ability to set default z-index for all jBox instances

I've been using jBox more lately and I have to explicitly set the zIndex option for each dialog instance as not to conflict with existing z-index values in my UI. Having to do this means that if my z-index order changes in CSS, I might need to update all my jBox instances accordingly.

A better option would be to allow me to set app defaults for all my jBox instances, like:

jBox.defaults.zIndex = 3000;