RecentChanges plugin now takes $days < 0 and

RecentChanges plugin now takes $days < 0 and
returns the oldest changes in the $days since the
beginning.  NB:  To return all changes, use
$days = 0.  Tested on Pear and adodb.
Implemented but not yet tested on dba
and cvs backends.

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1945 96ab9672-09ca-45d6-a79d-3d69d39ca109