Compute z-index where modals will be inserted

[saalaa]

This change ensures that the z-index of both the modal and its backdrop are computed according to the same CSS rules that will apply once the actual elements are added to the DOM.

This in fact fixes modals for people who use a prefixed version of bootstrap:

$.fn.modal.defaults.manager = '.bootstrap-css';

[JelmerD]

I see you have been struggling with the z-index as well. My problem is as follows: #239

I tried your fix if that solved my problem too but it didn't. Perhaps you have an idea on how to solve this properly since it looks like you have a better understanding of this plugin than I do.