Add `django.contrib.sitemaps` into contrib

[mikebryant]

As mentioned in #23, add support for django.contrib.sitemaps

[mikebryant]

@nanorepublica is there anything needed besides adding sites to INSTALLED_APPS?

[nanorepublica]

looking at: https://docs.djangoproject.com/en/1.9/ref/contrib/sitemaps/#installation it would also be the template configuration is correct