Use Sequel as ORM.

• Uses Sequel as an ORM.
• Invalid/non-existent records (e.g. "b123a") can no longer be instantiated; they raise an error upfront.
• Deleted records have their own class with access only to appropriate data/logic (e.g record_metadata).
• More/better documentation.

With the addition of Sequel, performance will be much slower than it had been unless optional sequel_pg gem is installed (data retrieval might take close to 2x the time it used to take). With sequel_pg installed, there is no significant change in performance.