Increment major version due to API change.

The new auth API changes are not backwards compatible, which I discovered trying to use the current master README against the 2.12 release.

[Semantic Versioning](http://semver.org/) demains that backwards incompatible changes result in a new major version. You might not be currently applying semantic versioning to this project, but I would strongly recommend it.

Alternatively, you could patch the code to continue to accept "user:pass" string as auth parameters. In this case you would only need to increment the minor version to 2.13, to indicate the presence of the new "auth" api.