FIX: typo - code double up
Set client IP on messages.
Fixes: #59
Renamed PaymentGatewayController::get_return_url to get_endpoint_url, and rearranged parameters.
Moved payment identifier from GatewayMessage to Payment.
This revokes an earlier decision to allow associating multiple payment attempts with one Payment model object. Now each payment attempt is a different Payment model object.

Relates to #49