ALE 0.6 - modes and difficulties for Atari games

[JesseFarebro]

This PR along with openai/atari-py#49 introduces modes and difficulties for Atari 2600 games in Gym. This was introduced in version 0.6 of the ALE and also outlined in the paper "Revisiting the Arcade Learning Environment: Evaluation Protocols and Open Problems for General Agents" by Machado et al.

I had previously discussed versioning in openai/atari-py#49 with @christopherhesse as this PR would rely on a new API in v0.6 which allows you to query and set the mode and/or difficulty.

The other thing to note in this PR is the removal of self.game_path. I don't understand why you call ale.loadROM in the seed function. To my knowledge, you don't need to set the random seed before loading the ROM. The seed only affects the RNG for sticky actions. In fact, most examples in the ALE repository set the seed after loading the ROM.
I had to make this change as it seems seed gets called twice and the second call would override any previously defined modes or difficulties (each call to loadROM resets the mode and difficulty to the default of 0).

To get this PR merged I think we should discuss:

1. How to handle versioning in Gym with respect to atari-py. It's probably necessary to check for setDifficulty and setMode in atari-py. If the newer API isn't found we can throw an error asking the user to upgrade atari-py.
2. Gain some clarity on the ale.loadRom call in seed. I don't see this breaking anything but it would be nice to get some clarity on why this existed in the first place.

[christopherhesse]

For 1), why not just require atari-py~=0.2.0 in gym?

[JesseFarebro]

@christopherhesse I agree, I'm not too familiar with pip's upgrade strategy but if we specify atari-py~=0.2.0 in gym will it indeed upgrade atari-py even though the major version hasn't changed?

We'll have to bump atari-py then I can reflect that change in this PR.

[christopherhesse]

Yeah it should work, the major version for a pre 1.0 release should be the second digit. For that requirement https://www.python.org/dev/peps/pep-0440/#compatible-release indicates that it would require atari-py>=0.2.0, atari-py==0.2.*.

[JesseFarebro]

Removing loadROM from seed ends up breaking determinism (I can see how since the ALE manages Stella's RNG after a loadROM call). Without looking too much more into it I just figured I'd leave the loadROM call in seed. This way I just set the mode/difficulty in seed.

I have tested everything and it's working as I expect. This is ready for someone to take a look at now.

[pzhokhov]

LGTM, let's merge this, I'll bump version and release the new pypi package.

[JesseFarebro]

Thanks for everything @pzhokhov and @christopherhesse!