Testing the license in python files.

* empty files are ignored (use case: `__init__.py`)
* new set of codes starting with `I` (compatible with `PEP8_IGNORE`)
* fuzzy test to recognize the GNU GPLv2 (or GNU GPLv3)

E.g., result on inveniosoftware/invenio@9b7e4b9
```
modules/bibclassify/lib/bibclassify_webinterface.py: 17: I101 copyright is missing
modules/bibsword/lib/bibsword_webinterface.py: 0: I101 copyright is missing
modules/bibfield/lib/functions/get_doi.py: 15: I101 copyright is missing
modules/miscutil/lib/__init__.py: 0: I101 copyright is missing
modules/miscutil/lib/plotextractor_output_utils.py: 10: I101 copyright is missing
modules/miscutil/lib/redisutils.py: 0: I101 copyright is missing
modules/miscutil/lib/logicutils.py: 34: I101 copyright is missing
modules/miscutil/lib/pep8.py: 24: I101 copyright is missing
modules/bibauthorid/lib/bibauthorid_bib_matrix.py: 0: I101 copyright is missing
modules/webcomment/lib/webcomment_washer.py: 0: I101 copyright is missing
```