installation: setup improvements

• Expands project description in place of "Invenio is Fun".

• Adds tests target to extras_require so that developers could install test-related dependencies via pip install -e .[tests].

• Improves PEP-8 compliance and code kwalitee.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>