org: document explicit vs. on-the-fly

* doc/org/tut50.org: New file.
* doc/org/tut.org: Add it.
* NEWS: Mention it.
* doc/Makefile.am: Add tut50.org, and download plantuml.jar when needed.
* doc/org/.dir-locals.el.in, doc/org/init.el.in: Activate plantum.
* HACKING: Mention the Java dependency.

doc/org/.dir-locals.el.in

@@ -26,8 +26,10 @@
 		      (org-babel-do-load-languages 'org-babel-load-languages
 						   '((sh . t)
 						     (python . t)
+                                                     (plantuml . t)
 						     (dot . t)
 						     (C . t)))))
+              (org-plantuml-jar-path . "./plantuml.jar")
 	      (org-confirm-babel-evaluate . nil)
 	      (org-babel-python-command . "@PYTHON@")
 	      (org-babel-C++-compiler . "./g++wrap")