Add authorized functions where missing in Github.Repos

Added authorized versions of `contributors`, `contributorsWithAnonymous`, `languagesFor`, `tagsFor`, and `branchesFor`.

Also changed the `| With authentication` bits in comments to just `With authentication`, since the pipe was showing up in the generated documentation.

After opening this I noticed that the use of `GithubUser` for authorization examples is no longer correct; changed these to use `GithubBasicAuth` instead.