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.
- Added :auto_assign_permanent_id accessor to Ddr::Models module. Ddr::Models.auto_assign_permanent_id is only set to `true` by default when Rails env is production.
Merge pull request #75 from duke-libraries/permanent-id

Implements permanent ID assignment with EZID as the remote identifier service