add sqlite db backend

README.md: ajout d'un paragraphe sur comment contribuer à Repanier

mysite/common_settings: enable serving static files in dev mode (hackish: why don't we use DEBUG=True for this kind of settings?

requirements: specify tablib version in order to have repanier working with python 3. This should be removed once openpyxl is up to date

mysite/common_settings.py: refactor default settings 2/2 (be explicit here not using config.options('DJANGO_SETTINGS'))

Ease local development