Add publish date filters to weblinks module

The weblink module was showing all weblinks and did not check for start or stop date.
This commit adds this filter.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29183

The same applies to Joomla 3.x branch.
PR for J!3.x: https://github.com/joomla/joomla-cms/pull/964
Tracker for J!3.x:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30505