Implements a new MobExtension plugin which adds 'follow', and 'chase'
mixins that allow the mob to follow behind another mob, and chase them
while keeping at a safe distance to fire projectiles. Also adds a new
public method 'raycast' to CollisionManager, for drawing a line through
the world using bresenham's line algorithm whille checking for any impenetrable
objects.
Adds a new mob extension plugin which creates a walk_to() method on
Mobs to allow walking to another entity (accounting for the size of the
entity) with an optional facing position.
Adds a mob-extension plugin, which allows creating modules as mixins to
be monkey-patched into the Mob class. Having a standard method of doing
this prevents confusing problems with mixin name conflicts, by allowing
the plugin to raise an error that 2 mixins with the same name exist.
Gary Tierney