Fix registered status check for .be domains

The Belgian DNS provider does not set the "Status" flag to "REGISTERED",
but uses one of these three values: "AVAILABLE", "NOT AVAILABLE" and
"NOT ALLOWED".

See: http://www.dns.be/en/faq/faq_technical#1671
Merge pull request #6 from stevenrombauts/master

Fix registered status check for .be domains