new block: progressbar

![2014-12-15_1407](https://cloud.githubusercontent.com/assets/1333220/5434521/c6f537de-8463-11e4-876b-3b12a42c86bf.png)

1) I'm not sure, but why do we need this private `_setBarWidth` method?
2) Setters usually return block's instance (see [select#setVal][1] for example)

[1]: https://github.com/bem/bem-components/blob/8451f9c7dc7be1f27d6a6dd1877411ff24c811b2/common.blocks/select/select.js#L118-L121