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.

docs/usage/sending_mail.rst

@@ -198,6 +198,9 @@ Most of the options from the Mandrill
 
         message.global_merge_vars = {'company': "ACME", 'offer': "10% off"}
 
+    Merge data must be strings or other JSON-serializable types.
+    (See :ref:`formatting-merge-data` for details.)
+
 .. attribute:: merge_vars
 
     ``dict``: per-recipient merge variables (most useful with :ref:`mandrill-templates`). The keys
@@ -209,6 +212,9 @@ Most of the options from the Mandrill
             'rr@example.com':    {'offer': "instant tunnel paint"}
         }
 
+    Merge data must be strings or other JSON-serializable types.
+    (See :ref:`formatting-merge-data` for details.)
+
 .. attribute:: tags
 
     ``list`` of ``str``: tags to apply to the message, for filtering reports in the Mandrill
@@ -245,12 +251,18 @@ Most of the options from the Mandrill
 
         message.metadata = {'customer': customer.id, 'order': order.reference_number}
 
+    Mandrill restricts metadata keys to alphanumeric characters and underscore, and
+    metadata values to numbers, strings, boolean values, and None (null).
+
 .. attribute:: recipient_metadata
 
     ``dict``: per-recipient metadata values. Keys are the recipient email addresses,
     and values are dicts of metadata for each recipient (similar to
     :attr:`merge_vars`)
 
+    Mandrill restricts metadata keys to alphanumeric characters and underscore, and
+    metadata values to numbers, strings, boolean values, and None (null).
+
 .. attribute:: async
 
     ``Boolean``: whether Mandrill should use an async mode optimized for bulk sending.