Spawnable ThrownPotion. Adds BUKKIT-2542.

Requires: https://github.com/Bukkit/CraftBukkit/pull/1031
JIRA ticket: https://bukkit.atlassian.net/browse/BUKKIT-2542

Modifies `ThrownPotion` interface to use new methods from CraftThrownPotion.

Allows splash potions to be thrown to a location, just like Arrows, Fireballs and such.