Add Suretax::Connection#post and return Suretax::Response

- Use VCR + webmock to stub requests and replay API responses
- Wrap Faraday response in Suretax::Response
- Deserialize Suretax API response body in custom response
Add note about handling errors in the API response
Use simplecov to generate test coverage reports
Ignore VCR fixtures
Test against response parts instead of whole response

This will keep the tests from failing on the transaction number