Slider: Add initValue

According to this thread http://stackoverflow.com/questions/25740609/jquery-ui-slider-set-handler-between-two-steps-as-default-value, i have added an initValue that can be set between 2 steps. after the first initialisation the initValue will never reached again because the "value" is added or subtracted with "step"

the initialisation look like this:

```
$( ".slider").slider({
                step: 10,
                min: 10,
                value: 35,
                initValue: 35,
                max: 60
});
```