Confusing that column_names for the entity includes id, created_at and updated_at

The timestamp columns aren't guaranteed to exist in an ActiveRecord class. Maybe we could have a method called something like `timestamp_attributes` that adds those attributes. Cf. `t.timestamps` in migrations.

As to the id column, we could probably just make this an explicit (in the README) requirement until it becomes a problem: your records are assumed to have an "id" primary key column.
