[certeorem] Need to allow deferred setting of members for Subscription

The Subscriptions class requires all its members to be set when constructing the Subscriptions object.
This is a problem in the scenario where the Subscription is created early in a spring context and the subscriber is in a UI (e.g. Swing UI) and does not know its callback url until some user interaction with the user. In this scenario what is needed is the ability to set the members (specifically the callbackPrefix member) after the Subscriptions object has already been instantiated by the spring context.

A patch has been tested and will be posted to crucible for review shortly.