Consider adding Organization as an actor

[justinwb]

Currently we support Agent and Application as distinct types of actors in the interoperability ecosystem. We should consider whether it is useful and/or necessary to add an Organization type.

Some thoughts on relationship:

• An Agent may belong to one or more organizations (either through direct link or via credential
• An Organization may be the author of a given Application

[elf-pavlik]

I see this one related to #80