Allow '0' in AUTO_CLOSE_PINGBACKS_AFTER, AUTO_CLOSE_COMMENTS_AFTER and AUTO_CLOSE_TRACKBACKS_AFTER

Comments are disabled with '0' and allowed forever with -1 or None.
Update docs ZINNIA_AUTO_CLOSE* settings
Merge pull request #212 from benjaoming/master

 Allow '0' in AUTO_CLOSE_PINGBACKS_AFTER, AUTO_CLOSE_COMMENTS_AFTER etc