[B+C] (a) Allow plugins to catch Thorns damage - BUKKIT-3505

## Problem
When damage is done by Thorns, an EntityDamageEvent is called with the attacker as the one wearing the Thorns armor and the defender being the one swinging / shooting. This causes problems for plugins like NoCheatPlus, as it has to add an exception for far-away damage done by a player with Thorns armor (in reaction to a bow shot) - theoretically, a malicious player could wear Thorns armor and send false hit-swings.

## Solution A
Solution A is to have CraftEventFactory check the translation string to see if it is "thorns".
**Pros**: Doesn't add a new NMS class, Changes CB code instead of NMS code
**Cons**: Depends on a translation string in an untracked file (DamageSource)

## Links
Bukkit: https://github.com/Bukkit/Bukkit/pull/776
Solution B: https://github.com/Bukkit/CraftBukkit/pull/1036
Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3505