add UpdateWebMapVersion from and update the folderID method

Incorporates Pull Request #22 which adds a function to roll back the specified version of web maps for compatibility with ArcGIS Explorer Online.

Also overhauls the folderID method. Previously, this required looping through all of a user's content until a match was found. This usually had the effect of creating a ton of new requests and adding a considerable amount of time to the script running time. I've recommended deprecating the old method `__getItemFolder__`.