set_mob_effect

Simple copy/paste from set_peffect
#2244 ptexp and set_ptexp do not have the correct getter/setter relationship

Because of ... weird bukkit behavior. Code is from Essentials plugin
https://raw.github.com/essentials/Essentials/master/Essentials/src/net/e
ss3/craftbukkit/SetExpFix.java and all credits goes to ementalo,
snowleo and khobbits.
Merge pull request #71 from MariuszT/master

#2244 and set_mob_effect