Added public methods `Ezid::Identifier#unavailable!` and `Ezid::Identifier#public!`
to effect status changes (addresses #2).
`Ezid::Identifier#unavailable?` accounts for possible reason (fixes #16).
Added public method `Ezid::Identifier#deletable?`.
Merge pull request #29 from duke-libraries/status

Added public methods `Ezid::Identifier#unavailable!` and `Ezid::Identifi...