Add support for non auto-increment id keys to relational databases

Currently, tenacity only supports using the auto-increment integer id field to link back to the row in the relational database.  However, not all tables have this field, and in addition, it may make sense to use another key in the table to link back to the row.

Tenacity should provide some way of allowing to user to specify which key should be used to retrive the row from the database.
