More flexible modal box centering

When you trying to change inner content by javascript, the margin-top position stays the same and window looks like uncentered. See screen from example:
![Screenshot from 2013-01-01 17:17:22](https://f.cloud.github.com/assets/1408154/37289/6b9b6bde-5426-11e2-8950-2aec72738206.png)

If you use css centering, you can avoid this
![Screenshot from 2013-01-01 17:17:27](https://f.cloud.github.com/assets/1408154/37290/6bd9170e-5426-11e2-94a5-a1bc6dc5078e.png)