Refs #13 - Introduces common django config then overrides it for all exi...

...sting django versions available today. No magic for now, new configs classes will be needed when releases will come.
How will you manage this SESSION_SERIALIZER change?
You cannot put it in the base.
A solution could be:

* add it in Django1_5_3ConfigurationSchema
* then make Django1_5_4Configuration inherit from Django1_5_3ConfigurationSchema, so we do not have to repeat it in every release >=1.5.3<1.6.
* then add it too in Django1_6ConfigurationSchema, because we do not want Django1_6Configuration schema inherit from Django1_5_3ConfigurationSchema.

I do not see how to avoid to repeat the declaration.