Add % KO column in statistics table, close #1181

This adds a new column in the statistics table, providing percentage of KO requests for each request or group.

The new statistics table looks like this : 

![charts-ko-percent](https://f.cloud.github.com/assets/1514127/523655/780fbe2c-c0cf-11e2-8a1d-502e8dd37ec1.png)