Airdrop contract(s) using SignatureBouncer

🎉 Description

It'd be cool if we had a simple Airdrop contract that used SignatureBouncer.sol to issue tokens (with emission variants like the crowdsale's Mintable/Allowance types) to those with valid signatures from a server.

Bonus points for a nodejs script that runs across a list of addresses and generates signatures for each of them.

More bonus points for a simple rails/express based api endpoint for generating these signatures which follows a flow like:

1. user signs some challenge data using metamask/hardware-wallet/whatever
2. user submits that signature to the server
3. server recovers the address from the signature
4. server checks to see if address is in off-chain whitelist
5. if so, server signs a SignatureBouncer hash allowing the address to call the Airdrop contract
6. user can then submit that signature to the airdrop contract and receive their tokens

• 🐛 This is a bug report.
• 📈 This is a feature request.

Read SignatureBouncer.sol and the crowdsale's Mintable/Allowance variants first. Scope the core Airdrop contract around valid server signatures and token issuance, then consider the separately described Node.js signer and Rails/Express API flow. Done means the requested contract behavior and any selected bonus tooling cover the issue's six-step signature and whitelist flow.