Added coinbase payout script
Update print statements to use print()
Use instant_buy

We don't keep any escrow in Coinbase, so we always need to buy BTC first
before we can send it along to users as a payout. We do have instant_buy
enabled. Our instant buy limit is $1,000 per week.
Coinbase script now fetches amount and fee from response
Added a note to bitcoin exchanges, and added int/string conversion for fees
Print JSON result if coinbase doesn't give a 200
deduct amount + fee from balance
Fix errors, check for negative balance
Changed note, indented block.
Added minimum payout as 1$
Bitcoin address always fetched from db
Gender neutralize log message
Merge pull request #2802 from gratipay/add-coinbase-script

Added coinbase payout script