Fix Django 1.7 incapabilities

* replace ugtext by ugtext_lazy
* replace get_user_model method by import
* setup django before tests run
* fix tests
Move populatedb command to a management command

Issue #200
Remove south, we don't need it

Issue #158
PEP 8
We are using "-" as well in payment url
Add missing auth application
Use User model not string from settings
Get or create categories in populatedb command

Issue #200
Add all words and "-" to payment url
PEP8
Merge pull request #199 from mirumee/django1.7

Django 1.7 compatibility