Fix items in item frames being shot off. Fixes #725

Fixes the following cases:
* A player shoots an item out of the frame with an arrow
* A player tricks a skeleton into shooting an item of an item frame

Previously, the item would pop off in both cases, now it remains intact.

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As the logic to determine which player was responsible for the damage event became quite big, I put it in a new method, getDamagingPlayer. I placed the method in the Helper class, I hope that it's the correct location.

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Tested on CraftBukkit Beta 1.7.9-R0.2 (build 3092).