Improved CSS based on peercoin.net

CSS was changed to be more like peercoin.net. I re-arranged some HTML elements to make this possible, and made some other minor fixes.

Take a look:

![pos-calc-new-css](https://f.cloud.github.com/assets/6946454/2466195/595c410e-afac-11e3-8f5d-692f10885dcc.png)