Throw EntityTargetEvent for EntitySlime targeting.

This will fix the issue with slimes not triggering the EntityTargetEvent. I realize that they may not target in the same way as other creatures but I doubt that it will cause any potential issues by having this in CraftBukkit.

This may need to be declined due to it spamming the event but I'm sure the spamming of it could be easily fixed.

Reference;
https://bukkit.atlassian.net/browse/BUKKIT-1408
https://bukkit.atlassian.net/browse/BUKKIT-2521