Fix despawning of mobs on chunk unload.

* Mobs were being treated as "special" because unarmored mobs spawn with an
  item (1xAIR) in their armor array.  The armor test in isSpecialMob() has been
  amended to ignore AIR.

* In onCreatureSpawnEvent(), the sheep cap was incorrectly looked up under the
  key "sheep", rather than e.g. "sheepwhite".  Access to the cap values is now
  through methods in all cases.

* An additional debug trace has been added in the case where a mob is exempted
  from removal when the chunk unloads because it is a "special" mob. This debug
  message is off by default, as with the others.

* Duplicated code to format the entity "type" and location (e.g.
  "sheepblue at (world,0,70,0)") has been encapsulated in a method.

* I've seen a mixture of tabs and spaces in NerdNu code.  I've come to the
  conclusion that using only spaces leads to properly formatted diffs on github
  and is therefore preferable (except in the case of CH code where a consistent
  tab-based style is already in effect).  Apologies for the pain in reading this
  one diff.