Adding Webhook support

A new class has been added to the SDK to support webhook authorization.

Webhooks allow you to receive notification as soon as a score or verification becomes available. The advantage of this is that you will not need to call the REST API provided by Lenddo at a regular interval until a score/verification become available.

The class will allow you to positively identify that the incoming request is legitimate by using the hawk authentication scheme.

Please refer to the webhook authorization documentation provided in the README.md for integration details. If you have any questions please reach out to your Lenddo representative.