Add support of the django versions 1.8-2.1

[vznncv]

The pull request contains the following changes:

• support of the django 1.7 or lower has been dropped
• support of the django 1.8-2.1 has been added
• the south migrations have been replaced by django ones
• the tests have been moved into a separate folder
• the tox and pytest were added to run tests
• the .travis.yml was updated (new django versions and tox integration)
• the structure of the setup.py has been simplified