Doesn't appear to work with private GitHub repos

[miketheman]

When running gitfaces against a private repo, RuntimeError is raised:

Traceback (most recent call last):
  File "/usr/local/bin/gitfaces", line 29, in <module>
    gitfaces.fetch(args.repo, args.outdir)
  File "/usr/local/lib/python2.7/site-packages/gitfaces/__init__.py", line 53, in fetch
    _fetch_github(git_names, gh_repo, out_dir)
  File "/usr/local/lib/python2.7/site-packages/gitfaces/__init__.py", line 136, in _fetch_github
    'Failed request to %s (code: %s)' % (r.url, r.status_code)
RuntimeError: Failed request to https://api.github.com/repos/MYTEAM/PRIVATEREPO/contributors?per_page=100&page=1 (code: 404)

An enhancement to this tool would be to allow adding an auth token to then run against private resources.