Don't close picker when it is right-clicked in Firefox

stopPropagation() doesn't prevent right-click events from bubbling in Firefox, which causes the picker to unexpectedly close when it is right-clicked. This can be fixed by ignoring right-clicks in the event handler (see these [before](http://jsfiddle.net/theodorejb/UfeVc/) and [after](http://jsfiddle.net/theodorejb/UfeVc/1/) JSFiddles).