Create function to verify webhook

[NicolasBonduel]

It would be great to have a function to verify the webhook payloads.
This way, users don't have to go through the hassle of encoding the body and checking it against the verification header.

Stripe has a function like that in each of their SDKs as well:
https://stripe.com/docs/webhooks/signatures#verify-official-libraries
https://github.com/stripe/stripe-node#webhook-signing

[NicolasBonduel]

Here is what the Stripe signature header looks like:

Stripe-Signature: t=1492774577,
    v1=5257a869e7ecebeda32affa62cdca3fa51cad7e77a0e56ff536d0ce8e108d8bd,
    v0=6ffbb59b2300aae63f272406069a9788598b792a944a07aba816edb039989a39

I like the versioning part of it as well.