Implements permanent ID assignment with EZID as the remote identifier service

Closes #59
Changes:
- "exception" string column added to events table for ActiveSupport::Notifications::Instrumenter
  exception info added to the event payload.
- minted_ids table dropped.
- New RDF datastream: "preservationMetadata"
- New RDF vocabulary: Ddr::Vocab::Preservation
- New index fields for permanent_id and permanent_url.