Play nicely with installations that do not bundle urllib3

Debian removes the urllib3 library from requests:

http://anonscm.debian.org/viewvc/python-modules/packages/requests/trunk/debian/patches/02_use-system-chardet-and-urllib3.patch?revision=27448&view=markup
Merge pull request #12 from dfc/bundledfix

Play nicely with installations that do not bundle urllib3