Update admin.widgets to use Truncator rather than truncate_words

Django removed `truncate_words` in commit
9f9a4cdecdea16c202292e9dee2444827abfae3a.  Adjust `admin.widgets` to use
the `Truncator` object found in `django.utils.text` instead.
add support for truncate_words in django <= 1.3
Merge pull request #281 from fusionbox/master

Fix for removed truncate_words function in django.utils.text