added Online Resources Only checkbox

I needed to make facets sticky across searches in order to get this to work properly. When a user has built up a search with facets they might want to limit further to online resources. When they click the checkbox and then submit the form again I think they would expect to keep their active facets, not have them reset.

I've left this branch running on gwdev-edsu.wrlc.org if you want to try it out there.

fixes #821