Python 3.3 compatibility

I've added python 3.3 compatibility.

* explicit relative imports
* force_text instead of force_unicode(except for django 1.3)
* response.unicode_normal_body instead of response.content

And python3.3 tox environment:

[testenv:py33]
basepython=python3.3
deps=
  django==1.5.1
  South==0.7.6
  coverage==3.6

Hope, that it'll be useful =)