Restore folder `javadoc/latest` for Javadoc.

This folder has been used for many years. There are links to Javadoc at
that folder (e.g. http://stackoverflow.com/questions/156503/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests/2935935#2935935 ).
Restoring the folder `javadoc/latest` will hopefully restore the
Javadoc pages in Google Search, too.
Merge pull request #1029 from stefanbirkner/javadoc-old-directory

Restore old Javadoc directory