I made a patch that will make astrometry index file downloading as easy as...

I made a patch that will make astrometry index file downloading as easy as clicking a button, at least on Macs for now.  I did send a patch with a rough draft of this yesterday, but I made several improvements over that patch.  I made it handle situations where there is no internet connection.  It brings up a dialog telling the user it cannot reach the server.  And because it now checks for an internet connection before attempting the download, that improves performance.  I removed the part with the config patch since that was already applied.  I disabled the downloader for Linux computers until we can resolve the read only folder issue. I improved the progress bar code.  I also simplified the code slightly.