change ugettext to ugettext_lazy

As calls to the translation function are located in code paths that are executed ad module load time, ugettext_lazy should be used instead. See https://docs.djangoproject.com/en/dev/topics/i18n/translation/#lazy-translations. This fixes import issue when used with other django apps that have utilities that import installed_apps such as Haystack.
Merge pull request #22 from aaronvanderlip/master

Use ugettext_lazy instead of ugettext.