new block: progressbar

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

- it's marked as readonly property in [`i-bem.js`](https://github.com/bem/bem-core/blob/6d8459f3146d592d2712475e585d56c026baea8b/common.blocks/i-bem/i-bem.vanilla.js#L171) base class;
- there are some perforce issues with deep mutable objects in JS;
- it's harder to predict the value of the exact parameter while reading / supporting the code (in general), as it's much easily to think about nested objects as immutables.