Returns a map Layer.

        Arguments:
        queryset -- QuerySet for Layer
        Keyword args:
        stylename -- str name of style to apply

def layer(self, queryset, stylename=None):
        """Returns a map Layer.

        Arguments:
        queryset -- QuerySet for Layer
        Keyword args:
        stylename -- str name of style to apply
        """
        cls = RasterLayer if hasattr(queryset, 'image') else VectorLayer
        layer = cls(queryset, style=stylename)
        try:
            style = self.map.find_style(layer.stylename)
        except KeyError:
            self.map.append_style(layer.stylename, layer.style())
        layer.styles.append(layer.stylename)
        self.map.layers.append(layer._layer)
        return layer