Add option for multiple agreements per node agreement

This may be a bit harder, but it would be nice to be able to add multiple agreements through the use of checkboxes on one node agreement.

Like this http://www.aviisha.com/sleep-test/legal-form

This would be dependent on #643410: Add option to have Checkbox instead of submit buttons

I don't have time to work on this now, but may in the near future, I will let you know if I start work on it.