[B+C] Add methods to get and set knockback strength in Arrow. Adds BUKKIT-5103
[B+C] Add setCritical and isCritical methods to Arrow.java, Adds BUKKIT-5113
[Bleeding] Add ProjectileSource interface. Addresses BUKKIT-1038, BUKKIT-1156
[Bleeding] Add setCharged and getCharged to WitherSkull. Adds BUKKIT-3060
[Bleeding] Add setSignalDestination and getSignalDestination to EnderSignal. Adds BUKKIT-XXXX