Added the DblClick event

Now it is possible to hook up dblclick event:

    $(container).bind("plotdblclick", function (event, pos, item) {
        // zoom out
        plot = $.plot($(container), datas,
                      $.extend(true, {}, options, {
                          xaxis: { min: null, max: null }
                      }));
    });
Merge remote-tracking branch 'mmajcica/patch-1'

Merging mmajcica's changes to introduce 'plotdblclick' event.