0.0.4

** New Features
* Herald Remote Shell: open a shell session on another peer (experimental)

** Improvements
* New Message format encapsulating headers and metadata
* Herald unlocks send() calls when the target peer is unregistered.
It also calls the errback of post() calls.

** Bug Fix
* Fixing synchronisation problem: XMPP transport blocked on
_on_disconnected (/issues/14).