Add private repo support through API token

This pull request adds a configuration option `api_token` that allows the app to pull from a private repo.

If this key-value pair exists and is not false, then it will append a Header to the HTTP request:

```
Authorization: token [api_token]
```

This authentication mechanism is documented in:
http://developer.github.com/v3/#authentication

To generate a Personal API Access Token, go to:
https://github.com/settings/applications

The announcement about this feature may be found here:
https://github.com/blog/1509-personal-api-tokens