Added "spawnboss" input in trigger_bossclass

Uses the gamemode sort function to turn the highest scoring zombie into
a boss.
Added 2 items to zombiesurvival.fgd

-Added spawnboss input to trigger_bossclass
-Added prop_weapon entity
Merge pull request #5 from BClark09/master

2 commits relating to trigger_bossclass and the .FGD
Merge branch 'master' of https://github.com/JetBoom/zombiesurvival