cleanup backends

the debug backend and the django-mailer backend are no longer
neccessary, the functionality is achieved by setting the correct
Django EMAIL_BACKEND setting.
Setting CAMPAIGN_BACKEND to a module name in campaign/backends is
deprecated, use a full module path instead.
add an AppConfig class for Django 1.7+
django.utils.simplejson no longer exists in Django 1.7+
update Readme and docs
switch migrations from South to native Django (1.7+) migrations