Feature/spring with code review

I have some general suggestions to improve the code base based on the spring - refactoring, as well as some bugfixes, such as removing static public variable to synchronize settings across classes, and updating all Maven plugin versions to the latest ones.

I also integrated two bugfixes by Benjamin which are currently in the other pull request - missed that one, sorry.

Feedback welcome!
If CORS is not activated, this throws a NPE. I recommend to add an assertNotNull statement.