See also: https://github.com/stapler/stapler/pull/39

This commit includes several changes to accomodate the above change in Stapler (moving away from 'stapler-class'), as well as to make Jenkins consistently use this same JSON field (moving away from 'kind').

NOTE: Stapler will continue to accept 'stapler-class' for backwards compatibility, and similarly this plugin will continue to pass 'stapler-class' for plugins that have keyed off of it.