Add the ability for rpc responses to be CC'd to requested groups.

This allows the requester to dictate where it expects a response without
the RPC service knowing anything particular about the requester.  For
example, a routing service can be created that only listens on a single
group but by using this mechanism, all messages it sends to the ESB can
be appropriately forwarded back to the router by adding the CC field.
Bump version to 0.4.0.
Merge pull request #25 from nubs/master

Add the ability for rpc responses to be CC'd to requested groups.