Weighted average of precission, recall and F1 score

[Karol Grzegorczyk]

Hi,

To get more statistics on classification results, in our project we 
slightly extended ResultAnalyzer and ConfusionMatrix classes, adding 
calculation of weighted average of precision, recall and F1 score. Do 
you think that it would be useful to apply this modification to the 
trunk? We can provide path (including unit tests).

Regards,
Karol