Remove duplicate mob spawn listener (SpawnListener and MobSpawnDisable were both basically the same).

Also update MobSpawnDisable to only disable certain types of mob spawns, not all. This allows, for instance, spawn eggs to work.

Fixes #125
Merge branch 'develop' into arena-caching

Conflicts:
	src/main/java/net/daboross/bukkitdev/skywars/listeners/MobSpawnDisable.java