Add plantuml

so I can make some sequence diagrams with org mode.  Here is a sample

 #+begin_src plantuml :file tryout.png
   Alice -> Bob: synchronous call
   Alice ->> Bob: asynchronous call
   Alice <-- Bob: call3
   Bob -> Mike: call4
   Alice <<-- Mike: call5
   Alice --> Matt: call67
 #+end_src

 # call org-export-as-html-and-open to run plantuml
 # see http://plantuml.sourceforge.net/emacs.html