Improved API — yep, one more time

I added to the API:
 - some methods to manage the arenas;
 - some others to interact with the spectator chat.

Also, I added `@since` tags in the javadoc, to help developers identify what is the minimal version of SpectatorPlus needed.

Finally, I generated the [JavaDoc page for this API](http://depot.carrade.eu/html/bukkit/doc/spectatorplus/) :) .
You should include it in the wiki page, either using my link (I can host it for you) or duplicating it on your own server (there is only two files, the one linked and the [stylesheet](http://depot.carrade.eu/html/bukkit/doc/spectatorplus/stylesheet.css), in the same folder).

P.-S. About the `@since` tags: I assumed the next version will be the version 2.0. I can change this if you want to release one more 1.9 (or 1.10?) version.