py3.3 support with django1.6 (py2.6 deprecated, use py2.7)

- support python3.3 (django>=1.6)
- python2.6 no longer supported

travis:
- update build link in README
- build matrix: py3.3 with django: 1.6 and django-trunk
- build matrix: py2.7 with django: 1.2.x,1.3.x,1.4.x,1.5.x,1.6.x, django-trunk

fix for django trunk (~1.7) (it complained on conflicting models in models_test and test_django_helpers)
Merge pull request #39 from julienaubert/feature/py3_support

py3.3 support with django1.6 (py2.6 deprecated, use py2.7)