function to create custom kill icons
Cleaning up Minegun.weapon - It's only ever set to the same value, but in three different places.
Revamped damage source system

All damage source IDs are now generated the same way, which removes the distinction between custom damage source IDs and built-in ones.
Turned DAMAGE_SOURCE_ variables into globalvars
Merge pull request #230 from Medo42/kill-log-api

Kill log api