Add mocks for backend or demo API

[jonasagx]

I can't check the test results without a clearbit key. There should be either:

• a demo API the clients can call and receive useless data, with the right structure though.
  or
• a mock backend in the tests

[davidlumley]

Thanks @jonasagx that's a great idea. I'm a bit swamped at the moment but pull requests are more than welcome!

[baldrich-randori]

I would also love a mock interface. The way the code is structured right now makes mocking difficult.

[heyweswu]

just put up a PR to allow for some flexibility in using this client with mocks, @davidlumley are you still one of the maintainers of this repo?