Set variableName for a java violation with an ASTFormalParameter node

Extends JavaRuleViolation to also set the member "variableName" when the
provided node is of type ASTFormalParameter.
Make ReportTree implement Iterable<RuleViolation>

With this change we can easily iterate over a ReportTree using the
for-each loop of Java 5 instead of using ReportTree.iterator().
Make Report implement Iterable<RuleViolation>

With this change we can easily iterate over a Report using the
for-each loop of Java 5 instead of using Report.iterator().
Also the @Override annotation has been added to Report.iterator() and
ReportTree.iterator() as PMD switched to Java 6.
Merge pull request #17 from acanda/IterableReportTree
Merge pull request #16 from acanda/ASTFormalParameterVariableName