Enable download statistics for Eclipse ESCET downloads and P2 sites

It would be nice to know how many people download our releases. See also:

• https://wiki.eclipse.org/IT_Infrastructure_Doc#See_download_statistics.3F
• https://wiki.eclipse.org/Equinox_p2_download_stats
• https://wiki.eclipse.org/Project_Download_Stats

We should enable this for downloads at download.eclipse.org (see also #35 (closed)). We should also enable it for our P2 update sites.