re-implement auto-configure code for ol.format.GML

so that people don't need to specify featureNS and featureType manually, see: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/ol3-dev/VoLY-miQy_E/h3BwwpY5t0cJ
Oh wait, please disregard my above comment. But when we get into `ol.format.GML.readFeatures_`, is this always going to be a `featureMembers` node? What if there is no `featureMembers` node?