First fork update, quite a bit more than I originally intended.

- Restructured project to be close to the Stripe.Net project layout
- Added the shell of all API endpoints
- Update the settings
- Created a new base class for API Services
- Added many more domain objects
- Added Async functions for most endpoints
Complete API Refactor

All MailChimp endpoints now supported, taking a lot of inspiration from
Stripe.Net
Updated Readme

Providing credit to original library and Stripe.Net for getting this
started and providing a great example of how to implement a complex
client library.