Spawnable ThrownPotion. Adds BUKKIT-2542.

Requires: Bukkit/Bukkit#766
Requires: Bukkit/CraftBukkit#1026
JIRA ticket: https://bukkit.atlassian.net/browse/BUKKIT-2542

Allows splash potions to be thrown to a location, just like Arrows, Fireballs and such.

Adds getter and setter for `ItemStack c` on EntityPotion.java.