NTLM support

https://github.com/savonrb/httpi/pull/80 brings in up-to-date NTLM support for HTTPI, so I figured I'd take a stab at putting in the appropriate hooks to do NTLM auth in a Savon client too.

This probably needs to wait for a new version of HTTPI to be released, however, so the dependency version can be bumped too. Happy to make that change and add it to this PR when the new version is out.