Backup users

Hiya! I've recently noticed my Chef 11 backups were lacking something - users! Seems we've created a few new people using Chef 11 and ended up with some functional admin users without a corresponding client key. Guess this is the new way forward.

Enclosed is a quick branch to backup and restore users (http://docs.opscode.com/api_chef_server_user.html). Only quirk is the password reset on restore, otherwise it works fine.

Seeing as Chef::User was introduced in Chef 11 I chose to disable users support within the plugin. Hopefully this works properly as I lack a Chef 10 server to test against. Happen to have one?