Enhance Javadoc descriptions

- added Javadoc to all the public methods
- fixed inconsistent indents
- added a blank line between Javadoc header and @param section
Merge branch 'master' of github.com:recruit-tech/document-validator into enchance-javadoc-message
add assertions
add paragraph test
add paragraph test
change to down the level to 2nd section in testBasicDocument
 fix bug handling Paragraph in MarkdownParser
Merge pull request #81 from johtani/improve-parser-test

Improve parser test
Merge pull request #77 from takahi-i/enchance-javadoc-message

Enchance javadoc
Fix a bug in link extraction
Fix indentation
Merge pull request #84 from takahi-i/fix-handle-wiki-link

Fix a bug in link extraction