tweak bootstro._start to add onStart call.

onStart for bootstro.js

        bootstro._start = function(selector)
        {
            selector = selector || '.bootstro';

            $elements = $(selector);
            count = $elements.size();
            if (count > 0 && $('div.bootstro-backdrop').length === 0)
            {
                // Prevents multiple copies
                $('<div class="bootstro-backdrop"></div>').appendTo('body');
                bootstro.bind();
                bootstro.go_to(0);
                // Add those 2 lines for onStart callback
                if (typeof settings.onStart == 'function')
                    settings.onStart.call(this, {idx : 0});
            }
        };