Facebook sync

This pull request contains an alpha version of the facebook sync. There is still work to do - like sending and receiving likes for comments. There is an open issue as well that received shared postings don't look like shared posting.

But mostly it appears stable.

There is no documentation by now. 

For testers:
You should think about activating this setting:

$a->config['fbsync']['do_likes'] = false;

If you follow a page with several hundred likes the system gets really slow.

The other changes are mainly at fbpost - so that it is possible to write comments and do likes to facebook.