Allow clients to request tokens from the mint

[garious]

People will want to interact with the public testnet. The slow way is to set up a validator and wait for it to pay out from transaction fees. We shouldn't make that the only way, because it requires making ports on that node accessible to the leader node, which is considerably harder than spinning up the client demo.

Instead, add a new executable solana-drone that does testnet airdrops. Clients should send it its PublicKey and the drone should give it some tokens, if there are any left in that time slice.

$ cat mint-demo.json | solana-drone

solana-drone should implement some form of rate limiting on IP addresses, such as 1 request per minute per IP. but this can be done in a later release