Add opt-in statistics gathering

[ewinslow]

For Elgg sites who want to help promote Elgg by being listed on elgg.org or supply metrics to the Elgg Foundation, it would be fantastic if we had an option to send back statistics of new installations. However, we've done this poorly before, so we'd have to do it right this time:

1. Opt in. Opt out is a privacy violation.
2. Graceful failure in case of no access to outside.
3. We may want to take special care to alert the admins of the sites listed on elgg.org whenever there is a security upgrade. Otherwise someone could scrape the list of Elgg sites on elgg.org and attempt to exploit them one by one whenever a security patch is out.
4. Don't bully the user into opting in.