Added coinbase payout script

Confirmed. I ran the script using your key, and I did get an error as expected:

```
[gratipay] $ honcho run -e defaults.env,local.env invoke bitcoin_payout -u whit537 -a 1 --api-key-fragment=deadbeef
Record(username=u'whit537', api_key=u'deadbeef', bitcoin_address=u'1HMaJmtuvbWh6fsY8VG6xeVjoJdEHQgJsd', balance=Decimal('147.16'))
Fetching bitcoin_address from database: 1HMaJmtuvbWh6fsY8VG6xeVjoJdEHQgJsd
Sending bitcoin payout for whit537 to 1HMaJmtuvbWh6fsY8VG6xeVjoJdEHQgJsd
Coinbase transaction didn't succeed!
{u'errors': [u"You don't have that much - please visit the Buy/Sell page to add more funds to your account.", u'Instant Buy is not enabled on this account'], u'success': False, u'transaction': {...}}
[gratipay] $
```