Bugfix, group number buttons with one slide, and callback function.

1. When you only show 1 slide at a time, the first group number button isn't there, so, when you have 3 slides, only button 2 an 3 are there
2. Added a callback method that will be executed after slide, so you can do something with the data from the active slide

e.g.:
$("#agendaCarousel").agile_carousel({
    callback: function(that){
        console.log(that.find(".ac_selected"));
    },
    .....
});