Added an option to updatelayers to check for 'advertised' status of GeoServer layers and skip if false.  Requires modified gsconfig.py
Change to --skip-unadvertised filter to accept 'None' values: in some cases there will be no 'advertised' property in REST API representations
Change --skip-unadvertised option help msg
Merge branch 'master' of github.com:GeoNode/geonode
Bump gsconfig version requirement to 0.6.4
Merge branch 'skip_unadvertised' into geoserver_updatelayers_dev