Add an explicit key to download mixin functions

In this proposed change, I've extended DownloadUrl and LoadImageFromUrl functions to accept a fixed key when storing data from a given URL.  Often I pull a resource "<someguid>.dat" for a game but it's shared data between any number of server instances. I don't care which of those servers the data came from just that I cached it - so the URL is "too precise" for my needs :) 

Fixes #170