Improve and document JSON serialization for Mandrill API

* Add some context to exceptions on unserializable
  values (addresses #89).
* Document need to format merge data
  (into something JSON-serializable).
* Add RemovedInDjrill2 DeprecationWarning.
* Deprecate blanket date/datetime serialization.

djrill/tests/test_mandrill_send.py

@@ -4,6 +4,7 @@ from __future__ import unicode_literals
 
 from base64 import b64decode
 from datetime import date, datetime, timedelta, tzinfo
+from decimal import Decimal
 from email.mime.base import MIMEBase
 from email.mime.image import MIMEImage
 import json
@@ -507,6 +508,16 @@ class DjrillMandrillFeatureTests(DjrillBackendMockAPITestCase):
         self.assertEqual(sent, 0)
         self.assertIsNone(msg.mandrill_response)
 
+    def test_json_serialization_warnings(self):
+        """Try to provide more information about non-json-serializable data"""
+        self.message.global_merge_vars = {'PRICE': Decimal('19.99')}
+        with self.assertRaisesMessage(
+                TypeError,
+                "Decimal('19.99') is not JSON serializable in a Djrill message (perhaps "
+                "it's a merge var?). Try converting it to a string or number first."
+        ):
+            self.message.send()
+
 
 @override_settings(EMAIL_BACKEND="djrill.mail.backends.djrill.DjrillBackend")
 class DjrillImproperlyConfiguredTests(TestCase):