[B+C] Implement InventoryPickupItemEvent. Adds BUKKIT-3798

This event is called when a hopper or hopper minecart picks up a dropped item.

Bukkit: https://github.com/Bukkit/Bukkit/pull/799
Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3798

This depends on Bukkit#797: #797
and on CraftBukkit#1061: Bukkit/CraftBukkit#1061