Fix iterator/list problems for Python 3 support
Fix str/unicode problems in Python 3

Changes:
- Use unicode_literals from the future for Python 2.6.5+
- Use six's text_type for proper unicode use in Python 2/3
Add Python 3 to tox and drop Django 1.2 and 1.3
Add Python 3 to travis and drop Django 1.2 and 1.3
Fix repr tests for Python 2 (no string comparison)
Note Python/Django version support in CHANGES.rst
Remove datetime imports unused by Django 1.4+
Update supported Python/Django versions in README