[WIP] maintainability improvments, created concise classes used by the gym

[mdreano]

First of all: thank you for this repo, it's been really a nice starting point for ! ❤️

Extracted some of the gym code in separate classes

• rewards.py: responsible for reward computation and holding reward state
• reader_pyboy.py: responsible for reading memory of pyboy
• baselines_utils.py: not so sure about this one, but anyway it holds some logic as to how to create the model

Some refacto on reward. Update and reading was mixed altogether, not easily maintainable.

• first update rewards and only after that read reward state to compute total reward
• impacte step function accordingly in the gym