Add EntityEquipEvent and EntityPickupItemEvent. Adds BUKKIT-3655

Split from GlowstoneMC/Glowkit#3
Original Bukkit PR: Bukkit/Bukkit#777
Additional changes: Documentation changes (minor)
Glowstone implementation: Not done

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Previously it was not possible to determine when, for example, a skeleton
picked up and equipped a helmet. This is a fairly important feature for
plugins that need to manage entities to ensure balancing in concepts such
as minigames.

This commit resolves the issue by adding the event required to determine
when entities equip items.