Clean up global MANDRILL_SETTINGS

* Clean up Djrill backend __init__
* Fold MANDRILL_SUBACCOUNT into
  global_settings logic
* Add some missing override tests
* Update docs

docs/usage/sending_mail.rst

@@ -103,24 +103,17 @@ Some notes and limitations:
 Mandrill-Specific Options
 -------------------------
 
-.. setting:: MANDRILL_SETTINGS
-
 Most of the options from the Mandrill
 `messages/send API <https://mandrillapp.com/api/docs/messages.html#method=send>`_
 `message` struct can be set directly on an :class:`~django.core.mail.EmailMessage`
-(or subclass) object:
-
-Most of these options can be globally set in your project's :file:`settings.py`
-using :setting:`MANDRILL_SETTINGS`. For Example::
-
-    MANDRILL_SETTINGS = {
-        'tracking_domain': 'example.com',
-        'track_opens': True,
-    }
+(or subclass) object.
 
 .. note::
-    ``merge_vars`` and ``recipient_metadata`` cannot be set globally. ``global_merge_vars`` is merged
-    (see :attribute:`global_merge_vars`)
+
+    You can set global defaults for common options with the
+    :setting:`MANDRILL_SETTINGS` setting, to avoid having to
+    set them on every message.
+
 
 .. These attributes are in the same order as they appear in the Mandrill API docs...
 
@@ -215,10 +208,6 @@ using :setting:`MANDRILL_SETTINGS`. For Example::
     Merge data must be strings or other JSON-serializable types.
     (See :ref:`formatting-merge-data` for details.)
 
-        .. note::
-
-        If using :setting:`MANDRILL_SETTINGS` then the message ``dict`` will be merged and overwrite any duplicates.
-
 .. attribute:: merge_vars
 
     ``dict``: per-recipient merge variables (most useful with :ref:`mandrill-templates`). The keys
@@ -244,8 +233,6 @@ using :setting:`MANDRILL_SETTINGS`. For Example::
 .. attribute:: subaccount
 
     ``str``: the ID of one of your subaccounts to use for sending this message.
-    (The subaccount on an individual message will override any global
-    :setting:`MANDRILL_SUBACCOUNT` setting.)
 
     .. versionadded:: 0.7