0.2.10-rc.2

Add Persistor as a component similar to Model to be instantiated directly or through a Mixin.
Add Postgres Mixin and Persistor.
Add contextmanagers to include both the pre and post argument parsing steps.
Push argument parsing details into the component classes and out of the mixins.
Push df_from_query and df_from_query_by_id into the Persistor classes to allow implementations to differ.
Eliminate unnecessary dict construction for dataframes.