Start / stop callback improvements [JIRA: CLIENTS-746]

[lukebakken]

See #128

If a user does not provide the callback function to various start and stop methods, they may run into a scenario in which no connections and nodes are available when executing a command.

The user may wish to do other processing while connections are being established, and then check if the client is ready. At this point in time, they must check the state of the client to do so which may not be intuitive.

• If the user executes a command while the RiakCluster object is starting, an error should be returned indicating it instead of what is observed in #128
• Provide IsStarted and IsStopped convenience boolean properties that check the node's state to provide a result

[lukebakken]

@mogadanez - please open that as a new issue.