Test suite updated to Django 1.4 + much needed performance and API fixes

Since I'm not sure if @alex is still actively working on `django-taggit` anymore I'm going to maintain a fork with some badly needed fixes. Here's the first pull request:

- `python setup.py test` for Django 1.4
- A performance fix when dealing with heavy datasets by @vdboor (https://github.com/hcarvalhoalves/django-taggit/commit/a165f9b8624312f62362d425b7587d318260f5bd)
- A bugfix for the lack of `related_name` argument on `TaggableManager` by @marcofucci (https://github.com/hcarvalhoalves/django-taggit/commit/edee4053c35ff394738a3314f04b476a592d186d)