Polling timer for operations should be adjustable

[rfrye-github]

In connection.py, class Connection, method _send_recv, lines 406-427 is a while True loop. This polls the API server for completion of an async operation. Line 412 is sleep(1) to sleep 1 second between http GET polls. It would be useful to make this an optional parameter passed in to init and various methods using _execute, default value 1 (retaining existing behavior) so that if running long operations in constrained environments cycles and traffic are not wasted constantly checking for "are we there yet?" results.