Changes:

• minor fixes (thanks to Gabriel Scherer),
• better error and warning messages (thanks to Gabriel Scherer),
• handle prefix in all xml nodes and attributes (upgrade to Xtmpl 0.5),
• global definitions now are of the form stog:foo,
• add: module system (not yet stable),
• add: new caching system, available for plugins too,
• add: Stog_plug.plugin_config_file,
• add: Stog_plug.register_level_fun_on_doc_list to register functions handling all documents at a time.
• fix: add '.' after counters in section title,
• add: when a basic template is missing, create one and issue a warning; these templates are: page.tmpl, doc-in-list.tmpl, by-keyword.tmpl, by-topic.tmpl, keyword.tmpl, topic.tmpl, by-month.tmpl.