add date filter for pseudo-fields

This allows code like:

$view->pseudofields['created_on']->past(1)->weeks()->rounded();
$view->fields['event_date']->from('2013-02-01')->to('2014-02-03');
$view->fields['estimated_finish']->past(1)->years()->future(1)->months()
;
finish all the filter field setting options

this is all untested, so don't use it until tests are written