[wallhaven] rewrite

- use API
- remove login support, add 'api-key' option
- remove support for "alpha" subdomain - alpha.wallhaven.cc used numeric
  IDs that can't be translated to the new ID system
- support direct links to wallpapers

scripts/supportedsites.py

@@ -115,7 +115,6 @@ AUTH_MAP = {
     "tsumino"    : "Optional",
     "tumblr"     : "Optional (OAuth)",
     "twitter"    : "Optional",
-    "wallhaven"  : "Optional",
 }
 
 IGNORE_LIST = (