Updated the postcard app requirements for story 0.0.
Updated the postcard app requirements for story 0.0.

1) Clarified the UI requirement for choosing the "to" field.
2) Will not be sending the public-key-hash via http headers. This is not needed as the authentication will be done when we move to TLS (story 0.6).
thaliproject/thali,thaliproject/thali,thaliproject/thali