add a "retrieved_at" method or attribute

[rbland]

I'm finding places in my software where it would be really handy to know when a record was retrieved from the database. For example, being able to tell if a record is old and needs to be reloaded or being able to tell if some data that was derived from a record needs to be updated. The attribute would just be a datetime object or time stamp and could be None for new records. This could be handled in a sub-class but it would be convenient if it was added to pyperry/base.

[tpett]

This would also be a great Processor candidate. Check out #16.