Added support for granting read access to non-github authenticated users.

We have been trying to use the copy-artifacts plugin with this plugin, and if you look in the code for copy-artifacts, at some point they create a UsernamePasswordAuthenticationToken and pass it to the ACL to check for "Authenticated" read access. This github-oauth plugin does not expect that, and the request ends up being denied. We were not sure if the original "allow read access" flag was intended to provide this too, so I added it as a separate option.