Send rewards directly to Btc, retain Coinbase as fallback

Implementing idea from mailing list:
https://lists.riseup.net/www/arc/whispersystems/2013-12/msg00009.html

Commit message:
Users can specify a 'Send-Bitcoin-reward: '-line in the commit message,
containing the address they want to get the payment delivered to. This
will be used as default. In the case the line is not supplied, money is
sent to coinbase e-mail.