Internal PuSH server for Friendica.

If you put this in your .htconfig.php:

    $a->config['system']['huburl'] = '[internal]';

Friendica will offer http://your_site/pubsubhubbub as a PubSubHubbub server instead of using e.g. the Google one. It handles subscriptions by adding them to the table push_subscribers, and if new posts have been created they are pushed out to subscribers when poller.php is running.

I have tested this on my small one user instance for a few days and it seems to work. I'm requesting a merge in order for wider testing to be done :-) This should solve bug #765.