Corrected Directory.unregister()

- Remove Peer bean instead of UID in names and groups sets
- **Do not** delete the 'all' group when empty
Log a debug trace when there were no peer to send a message to

(instead of a warning)
Corrected result of JSON-RPC remote service methods

- Added check for errors
- Now the method result is returned, instead of the whole result dictionary
By default, add the 'reply/' prefix to the subject of replies

- This can avoid infinite loops if a sender also handles the signal it emits
- No prefix is added if the reply subject is given
Special case for group 'all'

This group always returns the list of known peers, even if it is empty.
HTTP Transport: The sender UID is given in the POST headers

This will simplify the way the reception part works
HTTP Transport: Easier lookup of the sender UID

- Use the sender UID given in the POST request headers
- Check if the given sender UID and port match the ones known in the local directory