Use auto loading for the client

[christeredvartsen]

Would it be possible for you to use auto loading instead of require statements for the API client? Using require statements makes unit testing for us a lot harder when trying to mock the client.

Mockery supports mocking of static public methods for classes not yet loaded, but we can't use this feature because of the existing require statements, and therefore can't write suitable tests.

[christeredvartsen]

I have added support for autoloading using composer in my fork, and can send you a pull request if you want it.