[C] Spawnable ThrownPotion. Adds BUKKIT-2542

**The Issue:**

CraftBukkit was lacking support for spawning ThrownPotion entities either with `World.spawn` and `Player.launchProjectile`. This PR fixes that.

**Justification for this PR:**

Most projectiles could already be spawned anywhere on a world, **ThrownPotion**s had some issues with single/multi effect custom potions that just got fixed.

**PR Breakdown:**

Paragraphs breaking down what the PR is doing, in detail.

**Testing Results and Materials:**

I did a little unnamed plugin which listens on **ProjectileHitEvent** so when an arrow hits a something a custom splash potion is thrown to that location.

<https://gist.github.com/toqueteos/5222456>
Arrows shot with a bow spawn a splash potion with `World.spawn`.
Golden sword right click uses `Player.launchProjectile`.

*I just have Java 1.7 binaries (I'm sorry)*

**Relevant PR(s):**

CB-958 - <https://github.com/Bukkit/CraftBukkit/pull/958> - Already merged.

**JIRA Ticket:**

BUKKIT-2542 - https://bukkit.atlassian.net/browse/BUKKIT-2542