update extractor class hierarchies

- let the GalleryExtractor class inherit directly from Extractor
- make ChapterExtractor a subclass of GalleryExtractor
- change enumeration field names of GalleryExtractors to 'num'

test/test_results.py

@@ -27,6 +27,8 @@ TRAVIS_SKIP = {
 # temporary issues, etc.
 BROKEN = {
     "8chan",
+    "hentaifoundry",
+    "luscious",
     "mangapark",
 }