Improved API (arena support).

Also:
 - moved the messages related to the arena's lobby setup to the command executor;
 - added @since javadoc tags to every method of the API.
Improved API (added chat-related methods).
 - braodcastToSpectator;
 - sendSpectatorMessage.
Changed the visibility of the constructor of the API to protected.
Now, the only way to get the API is through the SpectatorPlus.getAPI() method.
Fixed javadoc (oops)
Changed the name of some methods.
Merge pull request #10 from AmauryCarrade/api

Improved API — yep, one more time
Update README.md