Resolves https://github.com/octokit/octokit.net/issues/677. Removes obscolete properties (gravatar). Makes Models.Response properties all be protected (most were already).
It's bothered me that our response models are mutable. They really shouldn't be. I made the properties have a protected setter instead of private to provide flexibility for others who might be deriving from these classes or testing them. Fixes #650
