More TNT fixes.

Chain-reaction TNTs are spawned in proper coordinates (FS #390)
Centralized Primed TNT entity spawning and made available to the plugins.
Internal changes for better TNT performance.

git-svn-id: http://mc-server.googlecode.com/svn/trunk@1604 0a769ca7-a7f5-676a-18bf-c427514a06d6