As per request by pnbecker and tdonahue, reworking supports() method to
be more restrictive in its handle validation while also still allowing
support for multi-instance environments with derived handles. Now, if a
handle does not match one of the currently known DSpace handle provider
formats, it will be rejected.
Updates the supports() method across the three relevant handle provider
classes to check if the identifier a) matches a valid handle starting
pattern or b) does not match a DOI starting pattern and does contain a
slash but is not an arbitrary URL.
