Changes:

• require Xtmpl >= 0.13.0,
• a document's DOCTYPE is now taken from the XML template corresponding to the document's type,
• install odoc_* files in stog ocamlfind directory,
• new option -pp for stog-ocaml-session,
• new exec rule to execute commands and include output in documents,
• new directory attribute for ocaml-eval to set the directory in which the stog-ocaml-session tool is launched,
• stog preview server: handle file dependencies to trigger recomputation of documents,
• now require lwt not only for server,
• change templates to use xhtml namespace,
• various fixes.