Add a function for validating payloads from GitHub webhooks

Given a GitHub secret, a payload and the signature delivered with the
payload in `X-Hub-Signature`, verifies if the signature is valid or not.
See issue #76.

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I wasn't sure what sample would you prefer for this, if any, as a working sample needs some kind of a web server. I could provide a sample application with [scotty](https://hackage.haskell.org/package/scotty) (around 20 lines of code), but it won't work without additional dependencies.
Since it is returning a boolean value, should we call the function validPayload or isValidPayload?