Docs: break apart the lengthy readme into organized docs

Also sphinx-ify where appropriate, and lots of general cleanup/editing.

docs/history.rst

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+Release Notes
+=============
+
+Version 0.3.0:
+
+* :ref:`Attachments <sending-attachments>` are now supported
+* :ref:`Mandrill templates <mandrill-templates>` are now supported
+* A bcc address is now passed to Mandrill as bcc, rather than being lumped in
+  with the "to" recipients. Multiple bcc recipients will now raise an exception,
+  as Mandrill only allows one.
+* Python 3 support (with Django 1.5)
+* Exceptions should be more useful:
+  :exc:`djrill.NotSupportedByMandrillError` replaces generic ValueError;
+  :exc:`djrill.MandrillAPIError` replaces DjrillBackendHTTPError, and is now
+  derived from requests.HTTPError.
+  (New exceptions are backwards compatible with old ones for existing code.)
+
+Version 0.2.0:
+
+* ``MANDRILL_API_URL`` is no longer required in settings.py
+* Earlier versions of Djrill required use of a ``DjrillMessage`` class to
+  specify Mandrill-specific options. This is no longer needed -- Mandrill
+  options can now be set directly on a Django ``EmailMessage`` object or any
+  subclass. (Existing code can continue to use ``DjrillMessage``.)
+