Improve accessibility (per Bootstrap recommendations) + update examples

Thanks so much for this plugin!  It's extremely useful.

These changes improve accessibility based on the recommendations of the Bootstrap documentation (http://getbootstrap.com/javascript/#modals):

> Make modals accessible
> Be sure to add role="dialog" to .modal, aria-labelledby="myModalLabel" attribute to reference the modal title,  and aria-hidden="true" to tell assistive technologies to skip the modal's DOM elements.

> Additionally, you may give a description of your modal dialog with aria-describedby on .modal.

This pull request adds `role="dialog"` and `aria-hidden="true"` to the modal.  It also assigns a unique id to the dialog title element (based on the dialog id) and adds an `aria-labelledby` attribute to the modal with the value of that title id.  Finally, it adds another config option ('description'), which if provided adds an `aria-describedby` attribute to the dialog for screen readers.