docs: enrich file headers with license statement

[tiborsimko]

Current status:

$ head -1 .travis.yml
# TODO: Add License header

Expected:

"Minified" license snippet.

[tiborsimko]

(1) In setup.py switch licence to GNU GPL, to have:

    license='GPLv3,
        'License :: OSI Approved :: GNU General Public License v3 (GPLv3)',

(2) Edit each source file and replace the beginning TODO comment with lines:

# This file is part of cernopendata-client.
#
# Copyright (C) 2019 CERN.
#
# cernopendata-client is free software; you can redistribute it and/or modify
# it under the terms of the GPLv3 license; see LICENSE file for more details.

[tiborsimko]

An example of how things will look like: https://github.com/reanahub/reana-client/blob/master/reana_client/errors.py