Updating set_visibility for embargoable items

Prior to this commit, if I would set the "Open Access with Embargo
Date", it would be treated as private; This is because the Sufia default
method #set_visibility is only aware of three values. I am intercepting
that value.

@WIP - I believe there is a logical inconsistency in the data validation
and open access.
Merge pull request #18 from jeremyf/update-set-visibility-for-embargoable-items

Updating set_visibility for embargoable items