Spawnable ThrownPotion. Adds BUKKIT-2542.

Requires: https://github.com/Bukkit/Bukkit/pull/766
JIRA ticket: <https://bukkit.atlassian.net/browse/BUKKIT-2542>

Allows splash potions to be thrown to a location, just like Arrows, Fireballs and such.

```java
ThrownPotion p = someWorld.spawn(someLocation, ThrownPotion.class);
// also:
// ThrownPotion p = (ThrownPotion) someWorld.spawnEntity(someLocation, EntityType.SPLASH_POTION);
Potion pot = p.getPotion();
pot.setType(PotionType.POISON);
pot.setLevel(2);
```

Tested and working.