#189 - 1.3.x - DSGeoServerAction support

Here comes DSGeoServerAction support for 1.3.x

For now, the geoserver-manager dependency was upgraded to 1.6-SNAPSHOT as it is master (to discuss possibly the next release).

This upgrade is required due to some fix to publish layer in Oracle NG datastores (see https://github.com/geosolutions-it/geoserver-manager/issues/71)