Fix Gitlab bug open time: only open bugs were counted. And set update time to min(update, close) for all bug trackers

First basic CI configuration

See merge request ow2/ow2-mrl!2

Need to be implemented with automated deployment using maybe SSH or Ansible.

Only compile and run test.

Minor pom.xml improvements

See merge request ow2/ow2-mrl!1

- Specify source encoding
- Configure Java version for source and target bytecode
- Specify (for `maven-compiler-plugin` and `maven-dependency-plugin`) and update (for `maven-jar-plugin`) plugin versions to ensure reproducible builds