Updated license URL
Annotations added and some cleanup.
Cleanup.
Added missing javadoc tags.
Replaced the empty expression with a better visibible one.
Null check added
(note: however,  i'm not sure if that special case may happen in reality)
Replaced the if-test with a return test.
Merge pull request #477 from offa/some_fixes

Some code improvements
Element.classNames() must not be cached
Test for Issue#478 (class names must not be cached)
Merge pull request #479 from hannibal218bc/fix-element-classnames

Element.classNames() must not be cached
Add CSS selector filter to HtmlToPlainText example