ElementProxy enhancements and fix for #65

New methods:
wait_for_absence(timeout = nil)
wait_for_presence(timeout = nil)

Fix for #65 

If the load_element is succeed, the check_absece will return false. However in the Waiter.wait_for will call load_element again and again until the time expired. I think we should return false and skip the wait_for if the load_element is succeed.