Added `Azure DevOps` (f.k.a. VSTS) receiver

[NecatiMeral]

Ported the payload classes from Microsoft.AspNet.WebHooks.Receivers.VSTS.

Haven't managed it to create a strongly typed example since the eventType is set in the body.

As far as I can see, strongly typed webhooks are only supported if the eventType can be determined from a header or parameter. Maybe going to add another way of detecting proper event types.

[dnfgituser]

All CLA requirements met.