Play nicely with installations that do not bundle urllib3

Debian removes the urllib3 library from requests. This patch allows responses to work with Debian's python-requests package. Relevant Debian patch removing urllib3: 

http://anonscm.debian.org/viewvc/python-modules/packages/requests/trunk/debian/patches/02_use-system-chardet-and-urllib3.patch?revision=27448&view=markup