Allow plugin spawning, dev build, fix chest NPE

* All plugin spawning can now be enabled so that other plugins are able
to spawn animals, monsters, etc (doesn't affect natural spawning).
* A NullPointerException when refilling chests was fixed.

Also, this commit introduces a file called
"latest-development-build.jar" which will always contain the latest
commits. That way, you don't necessarily have to wait for DevBukkit in
case of critical problems.