Fixes #1532.

The skeleton configuration file created by "nagare-admin create-app" doesn't contain all the possible parameters when their values are optionals.

But with this changeset, the `static` parameter, like all the others possible parameters, is now correctly available into the `config` object passed to the `set_config()` method:

  from nagare import presentation, component, wsgi

  class MyApp(object):
      pass

  @presentation.render_for(MyApp)
  def render(self, h, *args):
      return 'Hello world!'

  class WSGIApp(wsgi.WSGIApp):
      def set_config(self, filename, config, error):
          super(WSGIApp, self).set_config(filename, config, error)
          print 'Static directory:', config['application']['static']

  app = WSGIApp(lambda: component.Component(MyApp()))