Remove PaymentsRegistry and rename PaymentService

The payments registry is not longer needed.
Client is a more appropriate name for an API client instead of service.
Add httparty and dotenv gem

Use the dotenv gem to load config settings from environment variables
Use httparty gem for API communication
Use a module for configuration

A module is more suitable to share the same config in the amazon
namespace
Create the required request signature for AWS
Wrap the required and custom request params
Update documentation
Fix require statement for base64
Merge pull request #3 from mwallasch/amazon-api-methods

First amazon api methods and refactorings

Remove PaymentsRegistry and rename PaymentService
- The payments registry is not longer needed.
- Client is a more appropriate name for an API client instead of service.