posttroll.listener should use hierarchical logger custom

[gerritholl]

It is customary in python logging for loggers to have dotted names up to at least the module level, such as posttroll.subscriber. This idiom is not respected by the posttroll.listener module, which calls its loggers ListenerContainer and Listener. Those should be called posttroll.listener.Listener and posttroll.listener.ListenerContainer, respectively, so that user applications can set a logging handler applying to all messages coming from posttroll, and don't need to configure an extra logger for Listener. Another advantage of respecting the logging idiom is that users can see what module is issuing a log message, something that is unclear in case of a bare Listener.