[sankaku] rewrite/improve (fixes #44)

- add wait-time between HTTP requests similar to exhentai
- add 'wait-min' and 'wait-max' options
- increase retry-count for HTTP requests to 10
- implement user authentication (non-authenticated users can only view
  images up to page 25)
- implement 'skip()' functionality (only works up to page 50)
- implement image-retrieval for pages >= 51
- fix issue with multiple tags

scripts/build_supportedsites.py

@@ -82,6 +82,7 @@ AUTH_MAP = {
     "nijie"   : "Required",
     "pixiv"   : "Required",
     "reddit"  : "Optional (OAuth)",
+    "sankaku" : "Optional",
     "seiga"   : "Required",
 }