Rename CredentialProvider to RemoteYamlCredentialProvider

This should make it clearer when hardcoding the class becomes a problem.
Rename Repository to GitHubRepository

This class is (currently) hardcoded to work with GitHub. We should be
aware of that when we instantiate it.
Remove commented code

Always worthwhile.
Remove a comment
Merge pull request #9 from camelpunch/renames

Renames