More metrics on kills

[Ryan V]

Kills per Minute (# of kills divided by time since map started)
Kill Streak (# of kills before being killed)
% of Total Kills (total individual kills divided by total kills of the entire team)

[KevinSnyderCodes]

Some notes on this:

• For kills per minute (KPM) to be accurate, we'll need to explicitly know when the game begins and ends, and maybe even when the player becomes a warrior (KPM may be most interesting when it's calculated only during the time that a player is a warrior)

Everything else can be derived from the limited set of stats (i.e. only player kills and game start).