v1.20.0

PostNL-158: Add ERS (international return service)
PostNL-160: Add pick-up locations for Germany, France and Denmark
PostNL-243: Fix label size issue for international packets
PostNL-246: Fix pickup point selection for reorders
PostNL-248: Fix smart return issues
PostNL-249: Fix double return barcode for Shipping&Return
PostNL-250: Expand delivery code at the door feature for Belgium
PostNL-252: Add version number header to API calls
PostNL-258: Fix rounding up weights below 1 kilogram