Added functionality to send amount when settling transactions

@vcatalano  Hi man. Thanks for this awesome API. In my pull request I have added the functionality to send amount when we settle transactions. Please respond if you want me to make any changes.
 I wonder why you didn't add this functionality, on the other hand I am happy to have contributed to your API.

PS: No idea why the Travis build is failing. Maybe, because of connection problem with "authorize.net". It fails with error "socket.error: [Errno 104] Connection reset by peer"