Fixed a django 1.6 deprecation warning.
Fixed a Python 3 incompatibility, use force_text instead of force_unicode.
Fixed a Python 3 incompatibility when checking for string types.
Merge branch 'django-1.6'
Several more python 3 fixes.  Test suite now runs successfully with Python 3.4 and Django 1.6.
Fixed several deprecation warnings.
Added tox configuration and a test runner script.
Fixed a couple test errors when running under Python 2 that were introduced in the Python 3 fixes.
Separated out 1.7.x tests and dev tests (pulls from master branch).
Updated TagManager.usage_for_queryset to work with Django 1.7.x branch after lookup system refactor (https://code.djangoproject.com/ticket/16187).
Updated changelog.