Add hashcash

This is the "official" C implementation of [Hashcash](http://hashcash.org), often used in things like [scripts for mutt](https://pthree.org/2011/03/24/hashcash-and-mutt/).
Available [on FreeBSD ports](http://www.freshports.org/mail/hashcash/) – why not here?
I want to use it on my Macs too.