Norwegian translation

I've created a Norwegian translation of django-taggit (Norway has two written languages, this is for the most used variant called bokmål).  I've used the same language code (nb) as Django post 1.2 (https://docs.djangoproject.com/en/1.3//releases/1.2/#language-code-no).

This is my first time contributing on github, so please let me know if I should have done something differently.

-- Bjorn