Add functions to mimic the django messages API calls

To avoid importing and using the constants from django.contrib.messages,
I propose to add functions to mimic the way we can easily send messages in the
conrtib package::

   >>> messages.success(barry, "Barry, your report is ready")

(same with debug/info/warning/error)

PS: This is tested. And I also corrected an indentation problem as well as a var in a test