Wrong addition solved in add_faux_rows by adding parseInt

This solves https://github.com/ducksboard/gridster.js/issues/425.

The 
    fn.add_faux_rows = function(rows) {
        var actual_rows = this.rows;
        var max_rows = actual_rows + (rows || 1);

The second part was being added as a string before this patch.  This gave a huge row number and caused lots of lags.  Now it flies again.

I think also related to:

https://github.com/ducksboard/gridster.js/issues/340  (see comments of canfiax)