[twitter] added 'followers' extractor

modeled after the 'following' extractor

- cleanup
- add test

test/results/twitter.py

@@ -271,6 +271,12 @@ __tests__ = (
     "#class"   : twitter.TwitterFollowingExtractor,
 },
 
+{
+    "#url"     : "https://twitter.com/supernaturepics/followers",
+    "#category": ("", "twitter", "followers"),
+    "#class"   : twitter.TwitterFollowersExtractor,
+},
+
 {
     "#url"     : "https://twitter.com/search?q=nature",
     "#category": ("", "twitter", "search"),