Full implementation for OSX ready

Signed-off-by: avaid96 <avaid1996@gmail.com>

credentials/helper.go

@@ -9,4 +9,7 @@ type Helper interface {
 	// Get retrieves credentials from the store.
 	// It returns username and secret as strings.
 	Get(serverURL string) (string, string, error)
+	// List returns all the serverURLs of keys in
+	// the OS store as a list of strings
+	List() ([]string, []string, error)
 }