crawler: use urlparse.urljoin instead of string concatenation
crawler: use urlparse.urljoin instead of string concatenation

This avoided reporting 404s for links like ../other_page which browsers
& spiders will handle

acdha/django-test-utils,frac/django-test-utils,frac/django-test-utils,ericholscher/django-test-utils,ericholscher/django-test-utils,acdha/django-test-utils