Provide a way to verify binaries

[cornelius]

Bitcoin Core has a script to verify release binaries. This downloads a PGP signed list of checksums of the release binaries from two servers (bitcoincore.org and bitcoin.org) checks that the files are identical, checks the signatures, and checks that the checksums match the binaries. This is a way to verify that you have the right binary and nobody has tampered with it.

We need to figure out how we want to do this for unit-e. This is part of doing the first release (#735).

The current state of this script we have in unit-e can't work right now. We could also remove it for now.