Add API to retrieve list of contributors

[vbauer]

Now, I'm using the following workaround:

java

    public GHUser[] getContributors(final GHRepository repository) throws Exception {
        final Field field = GHRepository.class.getDeclaredField("root");
        field.setAccessible(true);
        final GitHub gitHub = (GitHub) field.get(repository);

        final GHUser owner = repository.getOwner();
        final String login = owner.getLogin();
        final String name = repository.getName();

        final Method retrieveMethod = GitHub.class.getDeclaredMethod("retrieve");
        retrieveMethod.setAccessible(true);
        final Object request = retrieveMethod.invoke(gitHub);

        final Method toMethod = request.getClass().getDeclaredMethod("to", String.class, Class.class);
        toMethod.setAccessible(true);
        return (GHUser[]) toMethod.invoke(request, "/repos/" + login + "/" + name + "/contributors", GHUser[].class);
    }

[kohsuke]

Implemented toward 1.62.