Db config proposal

@voisardf, here is what I would like to propose for creating a generic configuration of the db, thus the `crdppf\models.py` file.

This should resolve #67.

I'm actually not sure that defining a global variable is the best way to do it, but I can't figure out anything else. Moreover that's how we do it in the `sitn_c2cgeoportal` project.

Could you take a look at this proposal and tell me what you think of ? I will need quite a lot of time to do it for all classes, thus I would like to have your opinion first...

Take a more careful look at the lines following https://github.com/kalbermattenm/crdppf/compare/db_config?expand=1#diff-c527e86aa9718fbb94440204fc954a86R99

Thanks