Add support for reverse_lazy

[rclark72]

Right now, django-subdomains only support reverse(). I would like the ability to lazily reverse subdomain urls.

https://docs.djangoproject.com/en/1.7/ref/urlresolvers/#reverse-lazy

[squamous]

@lojack as a workaround you can implement subdomains_reverse_lazy in the same way that Django implements reverse_lazy using django.utils.functional.lazy