Make ruby-ntlm an optional dependency

Decided to bump the minor version number because this is technically a breaking
change in the API.

Fixes #107
Update Changelog
Trying to be friendly
Fatal log when Net::NTLM is not loaded, but was requested

Tip of the hat to @rogerleite for suggesting this improvement.
Merge pull request #108 from savonrb/ntlm-is-optional

Make ruby-ntlm an optional dependency