Support single selection lists

I love bootstrap-multiselect for multiple selection lists. I need almost the same functionality for single selection lists. This patch:
* Removes the code that auto-adds the multiple attribute
* Instead, saves a multiple=true|false option
* When adding listitems, uses radio buttons instead of checkboxes if options.multiple == false
* When selecting listitems, unchecks other items if options.multiple == false

Single selection lists want to look good too :grin:.