Add checksum/hash to download page

Nu is becoming a serious financial software. To making spoofing difficult I think the download page on nubits.com should have checksum or hash signatures displayed with a link to an announcement post in the forum that shows the same signature.

We’ll add the checksum and make it available in any notices of releases. We have a new binary coming out within the next day, so expect to see it appear on that release (0.4.6)