Added individual player scoring, and hit detection

Each player now has a score associated with them, while they are in the
game.  This score includes kills, assists and deaths.

A kill is when a player is the last to push a player before they fall
off a platform.

An assist is when a player applied a push to a player before they fell
off, but didn't get the kill.

A death is whenever a player hits the edge of the arena.

Added Targetable class to each player.  This stores all the players that
have pushed them, and appropriate timestamps.

Also fixed minor bug with Chat display on HUD.