STYLE: Document SlicerWizard/GithubHelper.py

Add documentation for the public API of SlicerWizard.GithubHelper. Also,
fix some code issues revealed in the course of writing the same:

- CredentialToken class is private; name should start with '_'
- logIn() function doesn't need a repository; it can be optional
- logIn() was not failing as expected in case of authentication failure
- getPullRequest() should accept additional criteria