Replace STATIC_URL by  static tag

I've replaced {{ STATIC_URL }} variable by Django 1.5 new {% static %} tag. Without %static tag, django-dashing required some special settings.

I would like ask you to merge this pull request.

Tested on Django 1.6, should work in Django 1.5  according to the [Django documentation][1]

[1]: https://docs.djangoproject.com/en/1.5/ref/contrib/staticfiles/#std:templatetag-staticfiles-static