Module Descriptors should define a scope at the handler level.

Technical document with background about PubSbub: https://docs.google.com/document/d/1L0qJ6VEmlKU4UCfznnc7du9qPZpjVq3DhcW68leIgEY/edit#

A scope will be multi valued property of an interface handler representing a concept of constraining or tagging the handler to particular use. The values of this property should not be limited to a predefined list of keywords, however, but instead allow for the free expression of multiple values.

Scopes will be used to support features in the proposed mod-pubsub, mod-workflow and mod-camunda modules.