Added 1.6 override annotation.
Fix checkstyle errors.
Fix java warnings and potential programming problems.
Add support for eclipse JDT compiler in tycho. To support new null analysis.
Add problem filter for addition of version constraint.
Update pom parent
Add sonar support
Update tycho version.
Merge develop-1.8 into develop-1.9
Update versions for 1.9.1
Update README for 1.9.1