Tox testing support

* Adds tox.ini to support easy testing in multiple Python
  versions.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Local git hooks support

* Adds support for integrating Invenio Kwalitee in local
  git repository hooks.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>