Establishing data sharing between Social Agents

[elf-pavlik]

This issue is intended to track an overall task included in my current project with NLnet

TODO

• Sharing access between social agents #238
• Use invitation to validate the source of an incoming request #214
• Refactor standalone Access Invitation specification #195
• authorization agent should check Resource Owner's address book if it includes requesting End-user's WebID #96
• Should we restrict Social Agent Reigstration discovery to AA of that social agent? #252
• Finalize design for Trusted Consents and Trusted Grants #187
• Imposing as a (social) agent #314