Store violation variable name in marker

Store the name of the variable that caused a violation (if any) in the
marker so it can be used by the quick fixes.
Update PMD to version 5.0.4 and Guava to version 14.0.1
Update plug-in version to 0.6.0
Merge branch 'master' of https://github.com/acanda/eclipse-pmd.git
Fix broken SingularFielQuickFix tests
Add support for remote PMD rule sets
Change return type of NodeFinder.findNode() to be Optional<>
Change return type of ASTQuickFix.getCompilationUnit() to be Optional<>
Change types of optional test parameters to Optional<>