Add django 1.7 support.

This pr includes:
- django 1.7 support in all codebase
- minor test refactoring for make it compatible with django 1.7
- generic test code improvements
- dead code elimination
- purge south migrations and replace them with new initial migrations from django 1.7
- update some third party libraries to new versions.

Note: django rest framework 2.4 has some strange behavior in serializers that not happens with 2.3.x and because of it, it isn't updated