Namespacing

Under the current dsl, namespacing table names for a specific environment such as `staging_2_events` would require loading an environment config in each model and explicitly setting the table name or monkey patching the underlying implementation.

It would be awesome if aws-record supported namespacing in an initializer or some other mechanism, similar to the way Dynamoid does so, for cases like ours where we're using a different set of table names for different environments. Thoughts?