Forum index improvements and more

+ option to add forum category description
+ option to show last user post avatar in forum
+ added global.css which will contain css rules that will help keeping
compt. with older themes
* forum index was reformated to be a bit more semantic and added more
css classes
* jscript.js has been beautyfied in order to facilitate further
modififications
Merge pull request #63 from JoiNNN/master

Forum index improvements and more