Create fingerprint() method and display its results

Fingerprints are meant be safer than actual public keys to display publicly on sites like github or twitter.  

This adds a simple fingerprint() method to both the client and Contact structs that computes the SHA-256 of the public identity key.  We display this fingerprint() method's results ahead of the public identity key and public key in showContact for both the GUI and CLI as well as showIdentity (CLI) and identityUI (GUI).  In addition, the other identity information fields are reordered to be more consistent between the CLI and GUI.

We should perhaps add a seed to the fingerprint() method, but I'll defer to others on that issue since doing so may require modifying both client/disk/clinet.proto and protos/pond.proto, and SHA-256 is very strong.