Add break-on-newline option

This changeset adds the option for "break-on-newline" which adds a <br /> tag on each newline inside a paragraph. This makes the library behave more like [GitHubs markdown](http://github.github.com/github-flavored-markdown/).
Update CHANGES and CONTRIBUTORS
Added img tag support for html_classes extra
Added newline at the end of test/tm-cases/html_classes.text file
Updated CHANGES and CONTRIBUTORS