Make ItemFactoryTest.java platform agnostic. Fixes BUKKIT-4695

Maven paths that include spaces (and possible other characters) get
improperly translated when using a file handle from a URL. This changes
the unit test to open a stream directly from the URL, providing proper
file resolution on multiple platforms.
Call ItemDeathEvent when an Item is killed. Adds BUKKIT-1900

This plugs a hole in a plugin's ability to track the lifecycle of Item entities, for example, the flag in a capture-the-flag game.
The method in CraftEventFactory performs a best-guess estimation of the DamageCause from the death. It may be prudent to merge this code and the logic in handleEntityDamageEvent sometime in the future.