Support 'Secret Token'

[kbrownlees]

Not sure if this is a new feature or not? Gitlab claims to be able to send a X-Gitlab-Token header with the request in order to 'validate received payloads'.

[spruce]

I don't quite understand what this issue is about.
Could you point me to some docs or so because I'm actually not actively using this project anymore. If you'd be able to create a PR I'd be really thankful. If you would need any help I would be glad to help.

[kbrownlees]

Apologies, https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/web_hooks/web_hooks.md#secret-token

Have been meaning to try and find time to make a PR, no luck so far..