Import hackage-security-0.6.0.0

The hackage security library provides both server and client utilities
for securing the Hackage package server
(http://hackage.haskell.org/). It is based on The Update Framework
(http://theupdateframework.com/), a set of recommendations developed
by security researchers at various universities in the US as well as
developers on the Tor project (https://www.torproject.org/).

The current implementation supports only index signing, thereby
enabling untrusted mirrors. It does not yet provide facilities for
author package signing.

(pho)