Create an API around a "Client" struct

[panicbit]

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).