fix timeSince function

This is a simplified version of https://github.com/concrete5/concrete5/pull/1093 that runs with PHP 5.2.
The key change is the removal of this line:
   $diff=$diff+86400; 
Also a check has been added if the date is older than one year. In that case the absolute date is returned because the calculation would be wrong otherwise.

This fixes: http://www.concrete5.org/developers/bugs/5-6-1-2/datehelpertimesince-returns-wrong-result-if-time-is-more-than-24/