Create an API around a "Client" struct #10
Description
An API that requires to instance and keep around a Client has a few advantages:
- More efficient
- Url only needs to be parsed once
- reqwest client can be reused (this is a significant performance boost)
- Better testability by allowing to point the client to a test / mock server (Mock recaptcha API in tests #6)
The current verify API can be kept for backwards compatability and also for getting a good quickstart with the crate (similar to how there is reqwest::get).