This commit adds logic to track all registered notifications that have
been registered by the client in a notification state when the default
automatic reconnect is enabled.
The notification state is then used to reregister for all previously
registered notifications on reconnect. This allows the caller to
continue receiving notifications across reconnect cycles.
Also, since the new package exposes more connection related error
information, add a new ErrInvalidEndpoint which is returned if the
specified enpoint does not appear to be a valid websocket provider and
only return the ErrInvalidAuth error when HTTP authorization failure
status codes are detected.
Closes#1.
Since the example illustrates callbacks for the OnBlockConnected and
OnBlockDisconnected callbacks, also register for the notifications with
NotifyBlocks.
While here, document the fact that most of the callbacks require
registration.
