* Add example atari_iqn_hl
* Factor out trainer callbacks to new module atari_callbacks
* Extract base class for DQN-based agent factories
* Improved module factory interface design, achieving higher generality
* Allow to specify trainer callbacks (train_fn, test_fn, stop_fn)
in high-level API, adding the necessary abstractions and pass-on
mechanisms
* Add example atari_dqn_hl