merge changes from monero: gitian (PR #4526)

[bleepster]

Merging the following commits:

• Prepare Depends Packages for Gitian Scripts (6d0ca4e25ef2a0962583697d6afe1fbf22aa44c8)
• Add gitian build script (c2f178902aedeb3e862dc1029c5e2531a18e42bd)
• Add checksums for download tools (f2127f9dca29c8b1e0a51bebeee5e8e49e6f4876)
• Change gitian.sigs repo from bitcoin-core to monero-project remote host (fed4e59886c3410429f9625caff0298ac7523bcb)
• Add windows descriptor to gitian descriptors (d147d24058ef4d4e25ff0603b6bc89f2d4095874)
• Add OSX gitian descriptor (9617fad0704dbca9b083409d5e7ef0319cf0806d)
• Adapt Readme and script to monero gitian build signing (8f96c718bc5b3844904b04868d5605778af41c39)

All commits are from PR #4546

See individual commit messages for more details.

[bleepster]

I will post a new PR for gitian with other updates.