Refactor the List and ListDir interface

Gives more accurate error propagation, control of depth of recursion
and short circuit recursion where possible.

Most of the the heavy lifting is done in the "fs" package, making file
system implementations a bit simpler.

This commit contains some code originally by Klaus Post.

Fixes #316

fstest/fstest.go

@@ -154,12 +154,13 @@ func CheckListingWithPrecision(t *testing.T, f fs.Fs, items []Item, precision ti
 	is := NewItems(items)
 	oldErrors := fs.Stats.GetErrors()
 	var objs []fs.Object
+	var err error
 	const retries = 6
 	sleep := time.Second / 2
 	for i := 1; i <= retries; i++ {
-		objs = nil
-		for obj := range f.List() {
-			objs = append(objs, obj)
+		objs, err = fs.NewLister().Start(f).GetObjects()
+		if err != nil && err != fs.ErrorDirNotFound {
+			t.Fatalf("Error listing: %v", err)
 		}
 		if len(objs) == len(items) {
 			// Put an extra sleep in if we did any retries just to make sure it really