i18n and German translation

This partially solves issue #54. Note however that it is currently necessary to put categories and categories.editor in INSTALLED_APPS to have categories.editor translated (see also #55).

**Note for further translations:**
In order to prevent duplicate translations it is necessary to ignore the strings in categories.editor when generting the translation files for categories. This can be done by passing the --ignore flag to makemessages, e.g.:

    django-admin.py makemessages -l de --ignore editor

**ugettext_lazy vs. ugettext_lazy**
I used ugettext_lazy for all the files. Maybe this should be discussed since I'm not totally convinced that this is a good idea.