introducing FindBugs maven plugin 

to monitor production code for bugs/warnings + added annotations to SuppressWarnings on all existing warnings in production code (no warnings is actually fixed in this commit) + set the maven property findbugs.skip to false by default so that TravisCI will execute it; still UnitTests are not yet covered by FindBugs

also added sonar plugin declaration to make it possible to use sonar with recent version of both maven and sonar (I should have done this one in a separate commit)