Add prove of App.net identities

App.net might a small, nerdy service those days but it would be great to verify my identies there here too.

Even more as some developers there (including myself) already think about combining the private messages with keybase.io based encryptions...

Start by clarifying the requested App.net identity verification behavior and how it should relate to the existing keybase.io encryption discussion. Then inspect the keybase client and website identity-verification entry points; done should include an agreed scope and a defined way to verify App.net identities.

Written by the indexing model from the issue text.