sync: Add Init and Sync functions

Init initializes the directory/repository before the first use.
Sync is called to unsure the directory is up-to-date.

admin/main.go

@@ -128,6 +128,9 @@ func main() {
 		sync.GlobalImporter, _ = sync.NewCloudImporter(cloudDAVBase, cloudUsername, cloudPassword)
 	}
 	if sync.GlobalImporter != nil {
+		if err := sync.GlobalImporter.Init(); err != nil {
+			log.Fatal("Unable to initialize the importer:", err)
+		}
 		log.Println("Using", sync.GlobalImporter.Kind())
 	}