[SOCIALGH-3] Use mixins for JSON mapping

As discussed via email with Craig, I plan to contribute some enhancements to the API binding. For example, I'd like to see whether or not a repository's a fork, list a user's organizations, etc. Before I started adding to the binding, I was conscious that by so doing I would be increasing the number of classes and properties that need to be updated to use mixins. Rather than adding to that task, I thought it better to take care of it first, hence this pull request.