Django urlconf callback

[sl0thentr0py]

One of our customers have a django-tenants multi-tenant setup. They have a dynamically resolved urlconf and set ROOT_URLCONF to an empty string which breaks this get_resolver in Django.

This results in a silent exception when we try to update the transaction name and hence they see Generic WSGI request for all their requests.

The solution to this is to expose urlconf in DjangoIntegration. The RavenResolver.resolve function already has this as an argument. I extended it to also allow the user to pass in a callback which these users need for their dynamic urlconf.

https://getsentry.atlassian.net/browse/WEB-530

[sl0thentr0py]

closing this because I believe I found a cleaner solution in
#1308