Add support for player / npc following

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.

data/plugins/entity/mob/following/plugin.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<plugin>
+  <id>mob-following</id>
+  <version>1</version>
+  <name>Following</name>
+  <description>Adds following for mobs.</description>
+  <authors>
+    <author>Steve Soltys</author>
+  </authors>
+  <scripts>
+      <script>following.rb</script>
+  </scripts>
+  <dependencies>
+    <dependency>mob-walk-to</dependency>
+  </dependencies>
+</plugin>