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

This event is called when a hopper or hopper minecart picks up a dropped item.

CraftBukkit: https://github.com/Bukkit/CraftBukkit/pull/1063
Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3798

This depends on Bukkit#797: https://github.com/Bukkit/Bukkit/pull/797
and on CraftBukkit#1061: https://github.com/Bukkit/CraftBukkit/pull/1061