TNT fixes.

Other blocks can be placed next to TNT.
Explosions activate nearby TNT blocks.
TNT doesn't destroy bedrock, obsidian and liquid blocks.
Server doesn't crash when a TNT (or other entity) leaves the valid Y range.

git-svn-id: http://mc-server.googlecode.com/svn/trunk@1603 0a769ca7-a7f5-676a-18bf-c427514a06d6