pep8 code indentation: core folder

[popadi]

I indented the code for all the .py files in the core folder based on the pep8 convention (except the "migrations" directory). I ignored the "lines to long" warnings (false-positives) because you can't always write a line of code with less than 80 characters.

@iulianR : codeclimate & travis CI are :+1: ; if you want, you can merge this.

P.S.: this is the same thing I commited a week ago, but I reverted it a couple of days ago to add the .yml file first.

[iulianR]

It's not absolutely necessary to do the same thing for the rest of the files :). It's ok if we just pay attention when committing in the future and fix problems as we go.