Instead of looking for a "secret" field hmac is used. Therefore the
raw payload is hashed with all secrets consecutively in order to
validate its content. If the content is certified the established
behaviour is pursued..
To create a minimalistic parser, nom is used to identify and replace
parameters given in the command field.
For clarity the `action` field for hooks was renamed to `command`.
In order to validate requests a field called `secret` has to be sent
containing a secret key which validates the request. A hook will be
executed only if the secret sent with the request matches the hook's
secret.
