Added new <extraClasses> parameter to javah target

This parameter allows processing classes like java.sql.Types that are no
actual *.class files in the <classDirectory> but come from the Java
runtime.
added documentation for extraClasses
Merge branch 'extra-classes'

I added a new parameter to the javah target to allow processing classes
like java.sql.Types that are no actual *.class files in the but come
from the Java runtime.

This resolves PR #22.