Implement an Authenticator which can fulfill a dns-01 challenge using
the Gehirn DNS (Gehirn Infrastructure Service) API.
Applicable only for domains using Gehirn DNS for DNS.
Testing Done:
* `tox -e py27`
* `tox -e lint`
* Manual testing:
* Used `certbot certonly --dns-gehirn -d`, specifying a
credentials file as a command line argument. Verified that a
certificate was successfully obtained without user interaction.
* Negative testing:
* Path to non-existent credentials file.
* Credentials file with unsafe permissions (644).
* Domain name not registered to Gehirn DNS account.
