pmd: verify #254 False+ : UnusedImport with Javadoc @link
pmd: fix #1032 ImmutableField Rule: Private field in inner class gives false positive
 Improved symbol table resolution to check inner classes, too.
pmd: switch to java 1.6
 we already depend on it, e.g. jcommander requires a java 1.6 JRE
pmd: fix NPE in ConstructorCallsOverridableMethodRule