Add statistics dialog

Closes #194
Will show all time UL/DL, ratio and some cache info.

P.S. All time UL/DL counters will overflow at 15,9 EiB

![qbstats](https://f.cloud.github.com/assets/1910559/1557324/7847acf0-4f0f-11e3-81a2-11b24bb1d8a8.png)