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/sync/file.go

@@ -19,6 +19,10 @@ import (
 type Importer interface {
 	// Kind returns information about the Importer, for human interrest.
 	Kind() string
+	// init performs the importer initialization.
+	Init() error
+	// sync tries to pull the latest modification of the underlying storage.
+	Sync() error
 	// exists checks if the given location exists from the Importer point of view.
 	exists(filename string) bool
 	// toURL gets the full path/URL to the given file, the Importer will look internaly (used for debuging purpose).