Further improve validation.

- Update README to include quick start section.
- Change `Dispatcher` logic to check for source directory at appropriate
  time.
- Use `Splitter` instead of split. Thank you Guava!
- Make `AbstractLintRules` consistent with Collections.unmodifiableList.
- Update tests to include source directory as file parameter (instead of
  the null).
- Split up tests into classes, provide them as a sample in README.