Simulating Atari environments

To get started with Gym, let's play some Atari games with it.

The Atari environments (https://gym.openai.com/envs/#atari) are a variety of Atari 2600 video games, such as Alien, AirRaid, Pong, and Space Race. If you have ever played Atari games, this recipe should be fun for you, as you will play an Atari game, Space Invaders. However, an agent will act on your behalf.