Added changed event to bootstrap-tabs.js

Created by: kasperp

This allows you to load and change tab content on request. e.g.

$('.tabs').tabs().bind( 'changed', function( e, c ) {
  // eventually unhook event handling from $(c.from)

  // load content from server and update $(c.to).html
  $.get( '/foo/' + c.to, function(data) {
    $(c.to).html(data);
  });
} );

Is meant to provide a very basic hook which allows you to load tab content with ajax.