Implement mint_ezid stub
Use Hydra::Ezid::Error for exception
Merge pull request #4 from jechols/implement-ezid-stub

Implement mint_ezid method to be mixed into model; raise an error if mint_ezid is called on an UnsavedDigitalObject
Put all mint_ezid logic into describe block
Merge pull request #5 from jechols/feature/reorg-mint-spec

Put all mint_ezid logic into describe block