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);
// Changes the colored aura on splash.
p.setType(PotionType.POISON);
// Single/Multiple effects can be added
p.addEffect(new PotionEffect(PotionEffectType.POISON, 2*60*20, 1));
p.addEffect(new PotionEffect(PotionEffectType.JUMP, 2*60*20, 1));
// could be made with:
// p.addEffects(Collection<PotionEffect>)
```

Tested and working.