new block: progressbar

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

тут надо добавить проверку на наличие content:
https://github.com/bem/bem-components/blob/v2/common.blocks/button/button.bemhtml#L51