v0.2 focusing on CartPole and fixing bugs

OpenAI Gym results:- https://gym.openai.com/algorithms/alg_zy3YHp0RTVOq6VXpocB20g
How to reproduce each run - https://gist.github.com/Sohojoe/7814a13dbae9fde1771606fe6081513d#file-duel_ddqn-v0-2-md