WebApp: Add support for predefined static files route.
render_template() should be method of WebApp, as it encapsulates search path.

Each web application would have own template search path, and that should be
encapsulated in WebApp object.

Also, templates are now by default searched in "templates" subdir, just as in
Flask.
Templates are now named by complete filename (with extension).
Templates are now by default live in "templates" subdir.