Implement Gateway and InitResponse.

In order to have a ActiveMerchant compatible interface that we can use in betterplace we implemented a Gateway and wrapped the resulting response in a tiny abstraction. Now it’s only necessary to expose the Gateway to perform a successful payment.
Updated gemspec after files were moved
Don't use spec/lib, move files
Some cleanup and refactoring

- Better error reporting
- Add some more specs to reach 100% coverage