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/com/pgcraft/spectatorplus/SpectateAPI.html) :) .
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 (Eclipse can generate these pages, or I can send an archive containing the files).

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.