Fixes #250 - Calling user specified change callback.

[maxschmeling]

I'm new to this library, but I had the issue of the user specified change method not being called. This seems to fix the issue.

I based the changed off of the code here:

        var touchMove = function (e) {
            var v = s.xy2val(
                        e.originalEvent.touches[s.t].pageX,
                        e.originalEvent.touches[s.t].pageY
                    );

            if (v == s.cv) return;

            if (s.cH && s.cH(v) === false) return;

            s.change(s._validate(v));
            s._draw();
        };