Add DuplicateSectionValidator
redpen-306 Duplicate Section Validator

- Rename DuplicateSectionValidator to DuplicatedSectionValidator
redpen-306 Duplicate Section Validator

- added misc methods (toString, equals, hashCode)
- added final to DuplicatedSectionValidator class
redpen-306 Duplicate Section Validator

- added misc methods (toString, equals, hashCode)
- added final to DuplicatedSectionValidator class
- added threshold parameter and the test
Merge pull request #358 from takahi-i/redpen-306

Add DuplicateSectionValidator