payment service HTTP client handling
Merge remote-tracking branch 'upstream/master' into service_notification
select payment tx will update payment fields
setting payment transaction should send notifications if requested
configs with explicit callback project key config, prepare notification
add callback config to init payment call
callback config checks on init payment
Go 1.2 compatibility, keepalives control introduced in Go 1.3
Merge pull request #67 from wongak/service_notification

Work on notifications