Added <extraClasses> parameter for javah

I added a new <extraClasses> parameter to the javah target to allow
processing classes like java.sql.Types that are no actual *.class
files in the <classDirectory> but come from the Java runtime.

Due to my lack of knowledge of GIT, this pull request seems to include 4 commits.
Only commit  3738771 and the added docs in 12a9353  are really needed!