Add pager support to Contribution Aggregate custom search

The "tutorial" for building custom searches instructs folks to use a copy of the Contribution Aggregate custom search as the base for developing their own searches:

http://wiki.civicrm.org/confluence/display/CRMDOC/Custom+Search+Components

However, this search is missing the code needed for pager support (rowcount / offset -> limit). We should add this functionality to this search (both for it's own sake and for folks who use it as an example).

Not sure if this conditional is sufficient - but in any case keep the implementation simple:
if ( $rowcount > 0 && $offset >= 0 )

{ $sql .= " LIMIT $offset, $rowcount "; }