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
Add support for Travis CI (http://travis-ci.org).

Simple CI for branches of JUnit forks. Now everybody can easily use CI for own branches. Pull requests are automatically build by the CI and the build result is visible on Github.