[B+C] Add broadcast and broadcastMessage to World. Adds BUKKIT-3638

**The Issue:**
There is no way (using the Bukkit API alone) that I can send everyone in a specified world a message.

**Justification for this PR:**
Mainly this PR contains a simple time saver for all developers out there. I know that doesn't sound like a good reason but it is just a lot nicer than using Iteration all the time.

**PR Breakdown**:
This PR adds two methods to the `World` class: `broadcast` and `broadcastMessage`. These methods are then implemented in the `CraftWorld` class.

**Testing Results and Materials:**
I made a plugin that uses these methods that you can download [here](https://www.dropbox.com/s/mgwgxzw6r44b4xv/WBT.jar?dl=1). It only has one command:

    /test [worldName] [permission]

**Relevant PR(s):**
B-881 - https://github.com/Bukkit/Bukkit/pull/881
CB-1170 - https://github.com/Bukkit/CraftBukkit/pull/1170

**JIRA Ticket:**
BUKKIT-3638 - https://bukkit.atlassian.net/browse/BUKKIT-3638