Simplify Mandrill webhook validation handshake.

Anymail was requiring Mandrill's webhook authentication key for the initial webhook url validation request from Mandrill, but Mandrill doesn't issue the key until that validation request succeeds.

* Defer complaining about missing Mandrill webhook key until actual event post.
* Document the double-deploy process required to set up Mandrill webhooks.

Fixes #46.

anymail/webhooks/mandrill.py

@@ -23,15 +23,23 @@ class MandrillSignatureMixin(object):
     def __init__(self, **kwargs):
         # noinspection PyUnresolvedReferences
         esp_name = self.esp_name
-        webhook_key = get_anymail_setting('webhook_key', esp_name=esp_name,
+        # webhook_key is required for POST, but not for HEAD when Mandrill validates webhook url.
+        # Defer "missing setting" error until we actually try to use it in the POST...
+        webhook_key = get_anymail_setting('webhook_key', esp_name=esp_name, default=None,
                                           kwargs=kwargs, allow_bare=True)
-        self.webhook_key = webhook_key.encode('ascii')  # hmac.new requires bytes key in python 3
+        if webhook_key is not None:
+            self.webhook_key = webhook_key.encode('ascii')  # hmac.new requires bytes key in python 3
         self.webhook_url = get_anymail_setting('webhook_url', esp_name=esp_name, default=None,
                                                kwargs=kwargs, allow_bare=True)
         # noinspection PyArgumentList
         super(MandrillSignatureMixin, self).__init__(**kwargs)
 
     def validate_request(self, request):
+        if self.webhook_key is None:
+            # issue deferred "missing setting" error (re-call get-setting without a default)
+            # noinspection PyUnresolvedReferences
+            get_anymail_setting('webhook_key', esp_name=self.esp_name, allow_bare=True)
+
         try:
             signature = request.META["HTTP_X_MANDRILL_SIGNATURE"]
         except KeyError: