Added formatLabel callback in options for "float"

The default formatLabel does prefix + value + suffix, so it's the same. Consumers of this plug-in can overwrite formatLabel to make custom labels. For example, if the value is 2.25 (hours), but you want that to be "2:15" (converted to hours:minutes), you can do that in the formatLabel:
.slider("float", {
    formatLabel: function(decimalTime) {
        var hour = Math.floor(decimalTime);
        var minute = (decimalTime % 1) * 60;
        return hour + ":" + ("0" + minute).slice(("0" + minute).length - 2);
    }
});