Plotting CartPole environment in Jupyter

[jamblejoe]

cartpole_env = ReinforcementLearningEnvironments.CartPoleEnv()
plot(cartpole_env)
plot()

Last plot() is needed to invoke plotting. GR.plot(env::CartPoleEnv) does return nothing, so no plot is shown in the notebook, I would fix but I dont know how GR works.

[findmyway]

You can simply call RLEnvs.plot(cartpole_env).

I'll work on #246 soon.