Adding sha3 digests to digest

[Alistair Crooks <agc%pkgsrc.org@localhost>]

This patch adds Taylor's sha3 code to digest:

http://ftp.netbsd.org/pub/pkgsrc/misc/agc/digest-sha3-20160217.diff

I also took the opportunity to update the licenses on my own code to
2-clause BSD.

All comments gratefully received - would like to get this in 2016Q1.

Thanks,
Alistair