Fix a NPE when a potion doesn't have a effect.

Signed-off-by: William Lebel <lebelwilliam@gmail.com>
Fixed a bug where all burnable materials has 100% change
- comparison was backwards
- non-burnable materials have an exception  with Random
Merge pull request #473 from greatman/fix

Fixed some potions NPE.
Better grass spreading using placement physic updates
Restore grass timer
Merge pull request #474 from fr1zle/master

Implemented basic Item dropping of mobs and animals.