Added an event to change the amount of experience an entity drops

Lex, you told me that there was a way to do this without an event you didn't told me what that was though :P

The experienceValue field is not respected by every entity and you also don't get access to the "original" experience value. 

The only other thing i can think of is to check for xp orb spawns and then reflect on the stacktrace to find the entity that drops them, that will cause problems though if multiple mods try to do that.