Add jacoco maven plugin for generating code coverage reports

I have added jacoco maven plugin to generate code coverage reports.

Reports are added to `target/site/jacoco-ut/`

I had to configure `maven-surefire-plugin`, so it's outside of `maven-enforcer-plugin` now.

I have run test to check how it affects running time, command used: `mvn package  -Dmaven.test.skip=true -Djacoco.skip=true` and difference with or without jacoco was unnoticeable 

When you choose to generate code coverage the overall build process will take few seconds longer.