Some bug fixes, code style following pep8 and updated Continuous Integration stages
- more robust search of the domain name in the Subject field of the CSR (thanks MatthaeusHarris)
- fixed a bug which required Contact to be filled (thanks adrium)
- fixed forgotten format for log message (thanks nurelin)
- support of CSR with SAN extension marked as critical (#9)
- code style updated to follow the python recommendations
- simplified regexp used to read the account key (#10)
- new Gitlab Continuous Integration configuration to build automatically docker images to run tests with always updated Debian Docker image (including Jessie, Stretch and Buster releases)