Issue105 + refactoring

[judgej]

Started as issue 105 only, which led to some refactoring for a proper fix. Then a bit more refactoring to remove some duplication mainly. Some features:

• All constants moved to a single interface, rather than be spread around the classes. So all constants should now be available from all classes and objects.
• Repeat authorize/purchase works on relatedTransactionReference as its source, rather than the multiple related* setters as before. The previous setter methods are deprecated, but still work. The documentation reflects the newer way of working with repeat transactions.
• Token/cardReference creation should now work consistently across the gateways.
• Additional documentation. There is still room for more.
• Deprecated ServerComplete* messages removed; use acceptNotification only for the server-to-server back-channel. The older ServerComplete* classes contained duplicate code that was getting a bit out of date as it was hard to keep it in sync with acceptNotification.
• The acceptNotificationResponse has been merged into the acceptNotificationRequest class. The new class has the combined features of both.
• Classes have not been moved. A future refactor will shorten the names and move messages into Shared/Server/Direct namespaces. This could then leave room to merge in the new Pi gateway and perhaps some of the Reporting and Admin API features.

[judgej]

Merging into master to get this into the main branch for pre-release trial. Nothing here should be a BC break, and we still support PHP 5.6 (no matter how much I would have loved to use the null coalescing operator). This will be made a minor release though, just in case the merging of the notification response into the notification request has some unforeseen issues, so the minor release will be a hreads-up. Also the removal of ServerCompleteAuthorize could bite some sites that have not moved over to acceptNotification (only acceptNotification has been maintained for some time).