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
@ahocevar AFAICT this works for both cases, i.e. topp-states-wfs.xml and topp-states-gml.xml as is shown in the test cases as well, is there another situation that I did not take into account?