Black formatter integration

[nahuelhds]

The Black formatter has been integrated to the repo and it has been ran over all the files.

Closes #66

Post-install instructions

Just execute

pre-commit install

That will install the pre-commit hook to your local repo so black and the other three hooks are executed before any commit.

I've added the instructions on the readme as well

[nahuelhds]

@edsu take a look at the .env test here. The problem was that I was creating the file in the root of the project. I've made it to be created in the test folder as well so it doesn't conflict with the Travis one. Anyway I didn't erase the @pytest.mark.skipif just in case. But maybe now it works even on Travis

[edsu]

Ok, I think I've got this merged into master. I'm testing without skipping the test on Travis now.