update version number and add changelog for version 1.6.0


git-svn-id: https://plugins.svn.wordpress.org/g-crossposting/branches/import-all-branch@995580 b8457f37-d9ea-0310-8a92-e5e31aec5664
translate new strings in admin settings to German


git-svn-id: https://plugins.svn.wordpress.org/g-crossposting/branches/import-all-branch@995581 b8457f37-d9ea-0310-8a92-e5e31aec5664