Support MS Exchange 2013 NTLM challenge

I have an MS Exchange 2013 server that likes to send this auth header as a challenge:

'www-authenticate': 'NTLM YadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYada, Negotiate'

I'm not sure why MS chose to break the protocol, but anyway python-ntlm chokes on the superfluous "Negotiate" at the end, and removing it lets me communicate with the server.
Pick only the NTLM challenge from the auth cookie
Strip spaces around multiple cookie values

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html section 14.47 only mentions comma-separation, not comma+space separation. So split only by comma, but clean up any spaces afterwards.
Merge pull request #33 from ecederstrand/patch-2

Support MS Exchange 2013 NTLM challenge