Publish Javadoc somewhere

[robinst]

The API documentation should be published somewhere, probably on gh-pages.

Should use a nice theme like doclava.

[robinst]

Also, we need to have more documentation on entry points (and package-info).

[robinst]

Done, it's available here: http://www.javadoc.io/doc/com.atlassian.commonmark/commonmark