Using Rails migration mechanism

Instead of the roll your own migrations, using Rails engine migration
methods. This prevents migrations from being run multiple times on an
application; Which in turn provides an upgrade path for those
installing Curate and then having new migrations "show up" in the
underlying engine development.

Closes #238
Requiring publisher validation for DOI minting

When we are remotely minting DOIs, the publisher is a required
attribute. Otherwise it is not required.

Closes #242