this patch implements those major features:
- `publish` is implemented on virtual pubsub service, thus XMPP entities can now publish
to AP using this service
- replies to XMPP items are managed
- `inReplyTo` is filled when converting XMPP items to AP objects
- `follow` and `unfollow` (actually an `undo` activity) are implemented and mapped to
XMPP's (un)subscribe. On subscription, AP actor's `outbox` collection is converted to
XMPP and put in cache. Subscriptions are always public.
- `following` and `followers` collections are mapped to XMPP's Public Pubsub Subscription
(which should be XEP-0465, but the XEP is not yet published at the time of commit), in
both directions.
- new helper methods to check if an URL is local and to get JID from actor ID
doc will follow to explain behaviour
rel 365
- HTTP signature is checked for incoming messages
- AP actor can now be followed using pubsub subscription. When following is accepted, the
node is cached
- replies to posts are put in cached pubsub comment nodes, with a `comments_max_depth`
option to limit the number of comment nodes for a root message (documentation will come
to explain this).
ticket 364
