Add Sunderer kills to the API

[Stardouser]

So that stats sites can include it in their data.

Number of sunderer kills/sunderer kill assist would seem (at least to me) an indicator of who is fighting to repel the enemy, and who is leaving the enemy sunderers up in an attempt to play TDM.

And I suppose if there could be some kind of sunderer repair data provided, it could help show how repairs are OP/too fast, especially when we've got 6 guys all stacking their repairs.