Send btcdconnected ntfns to connected clients.

If a websocket client was already connected and the wallet and/or
chain server is loaded into the rpc server (enabling the handlers
specific to those components), the btcdconnected notifications were
not being sent, and this could break clients that expected the
notification.  I'm not happy with this change, but since this is how
notifications are currently done (unsolicited), and to not break
compatibility yet, I'm adding these back in for now.

Eventually, this notification will require explicit registration
before it is received by a client.  See issue #84.

Closes #115.