[B] Update /say to vanilla behavior. Fixes BUKKIT-4224

**The issue**
Bukkit's implementation of the `/say` command doesn't match its vanilla counterpart. The following screenshots show the differences when running the command in the following ways:

1. From in-game
2. From the console
3. From a command block (un-named)
4. From a command block (named 'Test')
5. Remotely, using a [RCON utility](http://www.minecraftforum.net/topic/842376-minecraft-server-rconquery-utility/)
6. In-game without any arguments

Vanilla:
![Vanilla](http://puu.sh/3wiiT.png)

Bukkit:
![Bukkit](http://puu.sh/3wiea.png)

**PR Rundown**
This PR makes Bukkit's implementation of the `/say` command act exactly the same as the vanilla one, but in the dark purple colour. I would show you a screenshot but there really is no point :wink: 

**Justification for this PR**
To quote from CONTRIBUTING.md:
> One of the goals of the Bukkit project is to be an extended Minecraft vanilla server

This PR makes Bukkit one step closer to being an extended Minecraft vanilla server!

**Related Links**
Jira: [BUKKIT-4224](https://bukkit.atlassian.net/browse/BUKKIT-4224)