Support private repositories

[thSoft]

If I specify a private repo, importing fails with the following error:

Traceback (most recent call last):
  File "main.py", line 44, in <module>
    importer.import_issues()
  File "/Users/thsoft/Development/jira-issues-importer/importer.py", line 101, in import_issues
    self.import_issue_with_comments(issue, comments)
  File "/Users/thsoft/Development/jira-issues-importer/importer.py", line 119, in import_issue_with_comments
    response = self.upload_github_issue(issue, comments, headers)
  File "/Users/thsoft/Development/jira-issues-importer/importer.py", line 144, in upload_github_issue
    .format(issue['title'], response.status_code, response.json())
RuntimeError: Failed to POST issue: '...' due to unexpected HTTP status code: 404
errors:
{u'documentation_url': u'https://gist.github.com/jonmagic/5282384165e0f86ef105#start-an-issue-import', u'message': u'Not Found'}

It would be great if private repositories were also supported.