Major refactoring of async code

The result is correct handling of tiddlers being overwritten within
recipes.

node_modules/async/package.json

@@ -1,16 +0,0 @@
-{ "name": "async"
-, "description": "Higher-order functions and common patterns for asynchronous code"
-, "main": "./index"
-, "author": "Caolan McMahon"
-, "version": "0.1.15"
-, "repository" :
-  { "type" : "git"
-  , "url" : "http://github.com/caolan/async.git"
-  }
-, "bugs" : { "url" : "http://github.com/caolan/async/issues" }
-, "licenses" :
-  [ { "type" : "MIT"
-    , "url" : "http://github.com/caolan/async/raw/master/LICENSE"
-    }
-  ]
-}