[#16] admin: save-and-add-another action bug fix

Overriding the way how django creates response for "add" and "change"
actions was the simplest solution I've found. It is based on
django.contrib.admin.options.AdminModel implementation.

Closes [#16]
admin: app test added

Related to [#16] but also checkes if admin settings list is properly
displayed and checks if validation works.