Reordered some code into the notification package
Moved Queue into apns package
Renamed notification structs

Notification => header
NotificationFailure => Failure
NotificationAndPayload => Notification

Also serializing notifications can now be done simply by calling
n.Bytes()
Use Queue type instead of []Notification
Modified README for nice Queue API
Removed unused NotificationToBytes()
Don't use relative imports in non-local package