Add get_state() method to KazooServiceRegistry.

[diranged]

The get_state() method returns the current known conection state of the KazooServiceRegistry object to Zookeeper. This state is managed by the _state_listener() method and is updated any time a connection state change is detected.

get_state() also accepts an optional callback argument that will add a callback to the state listener and update that callback any time the connection state changes.

[mikhail]

Stylistic suggestions, but overall lgtm; +2