Implementation of the /spectate say <message> command

I used the style of the Essentials' private messages for the format of the message.

The result is like this. The pseudonym displayed is the one returned by `Player.getDisplayName()`, here in blue because another plugin colors the names according to the teams.
![spectatorplus_broadcasttospectators](https://cloud.githubusercontent.com/assets/1417570/3615487/3220aafa-0dc9-11e4-9c54-9e16387dead7.png)
I used the white color instead of the gray one because I think messages broadcasted to spectators are important messages (not a discussion or a comment, because the sender can't see any message except the one he sent).

```
/spec say Hey, this is a spectators-only message.
```

This fix #1.