Verify that all (base-) classes declaring @ClassRules are public
- Add the new method getDeclaringClass() to FrameworkMember
  - FrameworkMethod will return the class where the method is actually
    declared
  - FrameworkField will return the class where the field is actually
    declared
- Add validation logic to RuleFieldValidator
- Add test for validation logic
Merge pull request #764 from ferstl/classrule-in-public-class

Verify that all (base-) classes declaring @ClassRules are public