Non-id dereferencing

Added natural_key accessor and id_params to perform non-id dereferencing

- alias `set_natural_key` to `natural_key=`
- `id_params` determines which key to use (natural, falls back to primary)
- The show action now uses `id_params`
- The `natural_key?` provides a way to extend the strategy to determine when a `natural_key` lookup should be used
- documentation

Part of #28