Support for Payment Service API endpoint

[adityashedge]

1. Wrapper methods for Payment Service API
2. Attach Payment Service to Branding Theme via the endpoints below: a. GET /BrandingThemes/{BrandingThemeID}/PaymentServices b. POST /BrandingThemes/{BrandingThemeID}/PaymentServices
3. Parse XML and create appropriate Xeroizer::Record objects for Branding Theme as well as Payment Service API response.

[adityashedge]

With the current implementation, I have to parse the XML by myself. It also diverges from the existing syntax and structure of the gem where the code to add associations rests in the parent and is shared by including the Extensions module from the parent. I have rewritten the Payment Service API integration with the above points in consideration.

@waynerobinson @CloCkWeRX Would like both your inputs on this implementation.

TODO Add test cases for the above code change. Planning to do it once I receive your feedback.

[rjaus]

I will write release notes for this and include it in the gem release.