WFS features

Building on work from @dkillick, this PR adds very preliminary support for adding features from OGC WFS servers. It is limited to WFS features described in gml in terms of LineStrings, LinearRings. It is also quite slow as no caching has been implemented and the tests rely on a real server. On the positive side, it is reasonably quick if the extent is small, thereby limiting the number of geometries requested and potentially projected.
Shouldn't this kind of warning also be emitted when `srs is None`?