setup: licence statement fix

* Fixes licence classifier in `setup.py` that mistakenly read MIT
  instead of GPLv2+.

* Fixes licence statement in `tox.ini`.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
invenio_client: export of __version__

* Exports `__version__` in the top level `invenio_client` module.

* Amends the order of import statements in `__init__.py' in order to
  conform more fully to PEP 8 recommendations.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
setup: PEP 8 improvements

* Amends long line in the Python classifier list, making TravisCI PEP 8
  tests pass again.

* Amends import order to conform more fully to PEP 8 recommendations.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>