Update Webhook to return response as a struct

[zahiar]

This makes it easier for consumers of these methods to extract data easily without having to manually convert the interface into a matching struct.

Also added tests to cover the webhook functionality.

[zahiar]

New tests pass 😃

go test -v ./tests/webhooks_test.go
=== RUN   TestWebhook
=== RUN   TestWebhook/create
=== RUN   TestWebhook/get
=== RUN   TestWebhook/update
=== RUN   TestWebhook/delete
--- PASS: TestWebhook (3.15s)
    --- PASS: TestWebhook/create (1.24s)
    --- PASS: TestWebhook/get (0.64s)
    --- PASS: TestWebhook/update (0.64s)
    --- PASS: TestWebhook/delete (0.64s)
PASS
ok      command-line-arguments  3.376s

[zahiar]

@ktrysmt The previous goo.gl shortlink referenced an old & now broken page, so I created a new shortlink to the new page.

[ktrysmt]

LGTM!