Revert "Fix some tests"

This reverts commit f4e0904358c492a4462b3aca4f36f98142558ffa.
Revert "Produce indented HTML output"

This reverts commit 89212051491945a1189fa4563ca0cf973700dea9.
Add multimarkdown to the test script
Add a changelog
Fix some JRuby specific problems.

1. A nokogiri bug meant JRuby was indenting while MRI/Rubinius was not.
2. Another nokogiri bug breaks JRuby's handling of entity references, so now we output unicode instead of references where possible.
Update some tests
Compare documents using nokogiri-diff
Fix some more tests
More workarounds for broken JRuby Nokogiri