adding gitlab file

.gitlab-ci.yml

src/main/java/be/cylab/mongomailexample/Attachment.java

src/main/java/be/cylab/mongomailexample/MailRetriever.java

src/main/java/be/cylab/mongomailexample/MailSender.java

src/main/java/be/cylab/mongomailexample/Main.java