Pubsub subscribe

Joining the dots from friending, this patch adds Pubsubhubbub subscription support for v0.3 and v0.4 hubs.

The patch adds PuSH discovery and subscribe on follow/unfollow, this means that you can start getting notifications when your friends make a post.

Future enhancement:
* [ ] Add a native hub so it'll work in a corporate setting
* [x] ~~Work out how to PuSH private posts, incorporating our ACLs. This may require we get involved in the spec development, but I'm sure @benwerd can hook up a contact.~~  Publish is just a URL, so would be handled by our ACL system (if I understand things correctly)