Initial release, Ticket+Key --> NDEF mapping

We seem to be producing proper NDEF code now, based on extended typing as permitted by the NFC Forum.

The NDEF is produced from a Kerberos ticket and its key is encrypted to a keytab entry. There is no reverse direction, though it would be possible to make such a mapping as well.

Have a look at the demorun.md file -- it gives a good idea of what the code does without actually needing to run it.

The whole code is mostly a demo anyway -- the output includes a session key in plaintext, and its anti-climax is a hexdump of NDEF instead of a directly usable form. Still, you may enjoy that :-)