notifier clients should support Target destinations

[Ken Giusti]

Currently a Notifier accepts a list of topic strings to use as the destination for notification messages.

However a notification Listener subscribes for notifications using a list of Targets.

The issue is that a Listener my override the control exchange using the Target's exchange attribute. But it is impossible for the notifier to override the control exchange with the current API.

I propose to allow Target objects as well as strings in the topics parameter in the Notifier constructor.