#10 given an ID and a Contact, encrypt or decrypt a text message

Not to concern with storing the messages for this.

fc02cde7df can now do this, but the importing requires well-formatted keys, the demo data is garbage for testing, and it currently only works on the Cred.

Instructions for well-formatted keys added to the readme.

Preliminary version here: 62bfd9ee6d

But the demo contacts need their own public keys added.

Added keys for the demo contacts yesterday: bdcd8f273c

Now it needs a way to encrypt longer messages.