new block: progressbar

![2014-12-02_1609](https://cloud.githubusercontent.com/assets/1333220/5263031/9ae9b42e-7a3d-11e4-9a1a-76abcf21cc69.png)

Why do you write to `param`? I think it is bad practice.
It would be better if you have a private variable `_val` for this purposes.