[photobucket] improve subalbum extraction (#117)

The former implementation would produce a complete list of all subalbums
for each (sub)album extraction. This would for example result in a
level 2 subalbum getting "extracted" twice: once through the root-album
(level 0) and once through its parent album on level 1.

In the current implementation only the next level of subalbums are
returned, which themselves will handle their next level in a recursive
fashion.

gallery_dl/extractor/photobucket.py

@@ -30,8 +30,12 @@ class PhotobucketAlbumExtractor(Extractor):
         }),
         ("http://s271.photobucket.com/user/lakerfanryan/library/", {
             "options": (("image-filter", "False"),),
-            "pattern": "http://s271.photobucket.com/user/lakerfanryan/library",
-            "count": ">= 22",
+            "pattern": pattern[0],
+            "count": 1,
+        }),
+        ("http://s271.photobucket.com/user/lakerfanryan/library/Basketball", {
+            "pattern": pattern[0],
+            "count": ">= 9",
         }),
         ("http://s1110.photobucket.com/user/chndrmhn100/library/"
          "Chandu%20is%20the%20King?sort=3&page=1", None),
@@ -74,23 +78,17 @@ def images(self):
             params["page"] += 1
 
     def subalbums(self):
-        """Yield all subalbum URLs"""
+        """Return all subalbum objects"""
         url = self.root + "/component/Albums-SubalbumList"
-        params = {"albumPath": self.album_path, "json": "1"}
+        params = {
+            "albumPath": self.album_path,
+            "fetchSubAlbumsOnly": "true",
+            "deferCollapsed": "true",
+            "json": "1",
+        }
 
         data = self.request(url, params=params).json()
-        albums = data["body"]["subAlbums"]
-        albums.reverse()
-
-        while albums:
-            album = albums.pop()
-
-            subs = album.pop("subAlbums")
-            if subs:
-                subs.reverse()
-                albums.extend(subs)
-
-            yield album
+        return data["body"]["subAlbums"]
 
 
 class PhotobucketImageExtractor(Extractor):