Allow overriding of icon.png's

In order to provide further extensibility, it would be nice if the user could elect not to update the icon with the ones on the server.

This may go hand in hand with whatever caching implementation is decided on, see #6

So it turns out this is pretty different than caching was implemented.

I'd probably like to see this done by creating a touch file per app, and if the file is present, don't download a new icon.png.

That would be nice