New feature: local web caches

I checked in a revision with a new feature last night: the ability to use local web servers as proxy caches for items. This means you can provide a http url to the "--set-cache-folder" option, and when InstaUp2Date can't find the file locally it will try addressed based on that one before looking out to the web to download the item. While this might not make much difference to most people, there are a couple of cases where this might have a real impact: 1) Consultants who may have multiple builders that are getting their catalog files remotely. With this setting up a builder can be just a few commands, and with a little work the local web cache can avoid the requirement of multiple computers downloading large updates from the internet. 2) People who are making hardware-specific images on new hardware and so need to get InstaDMG running more quickly. I am sure that there are more cases out there for this, but it is a start. There are a few other features that I want to get in over the next week to pair up with this, but we will see how fast I can get them done.