Core/Commands: implement pvpstats command

This is a very simple command to let the player quickly see which is the faction which won most BGs in the last 7 days and have an idea of the server balance.

```.pvpstats``` will output:

![pvpstats_command](https://cloud.githubusercontent.com/assets/75517/4779667/87455cec-5c3e-11e4-9716-682d0528ec1d.png)