Simple 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)

Minor thing:
could be simplified to
```sql
+DELETE FROM `trinity_string` WHERE `entry` IN (80, 81);
+INSERT INTO `trinity_string` (`entry`, `content_default`) VALUES 
(80, 'BattleGround victories in the last 7 days\nAlliance: %d\nHorde: %d'),
(81, 'BattleGround scores storing is disabled!');
```