Add helper interfaces commonly seen in ActiveMerchant.
Remove duplicate methods that are already defined by mapping() call.
Due to method name collision, the semantic of return_url has changed.
See link for common AM interfaces:
https://docs.google.com/spreadsheets/d/13i41t3ZZ_LoW59ayt1C9oy-3XGWhArVugWuCKDDpnKo/edit?usp=sharing
Add transaction_id (id from gateway side)
Add acknowledge and complete? for notification to do common checks.
These two are very common in AM implementations.
Do not remove CheckMacValue when doing checking.
So we can check again and it will still return the same result.
Basic upgrade notes
Merge pull request #6 from lulalala/compatibility

Compatibility changes