Fix a minor typo.
Separate login from subscribe with ESB messages.

This provides a better API for subscribing as you can now call it
multiple times.

Note that this does remove the ability to subscribe to multiple groups
at once.
Only subscribe to channels once.

This allows for lazy subscribing in client code without impacting
performance beyond the first message.
Auto-subscribe for the RPC calls.

There is no need to force the user to remember to subscribe to groups
for using the RPC methods - the client can do that for them and make it
even nicer to use.
Bump version to 0.3.0.