Fixs on_load not being called in TableScreen

table_view_controller was missing a loadView implementation and since on_load is [called explicitly in the spec](https://github.com/clearsightstudio/ProMotion/blob/master/spec/unit/tables/table_screen_spec.rb#L7) it couldn't fail the tests. Sorry I didn't spend much time refactoring to get rid of the explicit call to on_load, couldn't tell if there were other reasons for it to be there.