Serve tracker pixel from Tangent's infrastructure

This is to enable some coarse metrics of who is using Oscar.  We just
look at the referer header to get a rough idea of which sites are using
Oscar.

This is an important measure for Tangent, Oscar's sponsor.  It's
documented in the release notes and we'll make sure everyone knows when
Oscar is released.  It's easy to opt out of.

Fixes #656

docs/source/releases/v0.6.rst

@@ -52,12 +52,16 @@ Other notable new features include:
 
 * `New helper methods`_ in the ``EventHandler`` class for order processing.
 
-* `Reworked seach app`_ with support for easy faceting.
+* `Reworked search app`_ with support for easy faceting.
 
-.. _`Oscar now supports Django 1.5`: `Django 1.5 and custom user model support`_
+Also, to help justify Tangent's sponsorship of Oscar, 
+a simple `tracking mechanism`_ has been introduced.
+
+.. _`Oscar now supports Django 1.5`: `django_support`_
 .. _`Partners can now have addresses`: `Partner addresses`_
 .. _`Customer wishlists`: `Wishlists`_
 .. _`New helper methods`: `Order processing changes`_
+.. _`tracking mechanism`: `Tracking Oscar sites`_
 
 What's new in Oscar 0.6?
 ========================
@@ -183,6 +187,8 @@ Further reading:
 
 * :doc:`The demo site </internals/sandbox>`.
 
+.. _django_support:
+
 Django 1.5, 1.6 and custom user model support
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -249,6 +255,17 @@ Order processing changes
 
 There are changes to order processing methods
 
+Tracking Oscar sites
+~~~~~~~~~~~~~~~~~~~~
+
+Oscar's dashboard now serves a single pixel image from one of Tangent's
+servers.  This is included to gather information on which sites use Oscar,
+which is an important metric for Tangent, who sponsor Oscar's development.
+
+It can easily be disabled by setting ``OSCAR_TRACKING=False``.  If you do opt
+out, please email the mailing list with any production Oscar sites you are
+running.  This will help to ensure investment in Oscar's future.
+
 Minor changes
 ~~~~~~~~~~~~~
 

oscar/core/context_processors.py

@@ -10,5 +10,8 @@ def metadata(request):
             'shop_name': settings.OSCAR_SHOP_NAME,
             'shop_tagline': settings.OSCAR_SHOP_TAGLINE,
             'use_less': getattr(settings, 'USE_LESS', False),
+            # Whether to use a tracker gif in the dashboard to call back to one
+            # of Tangent's servers.
+            'call_home': getattr(settings, 'OSCAR_TRACKING', True),
             'google_analytics_id': getattr(settings,
                                            'GOOGLE_ANALYTICS_ID', None)}

oscar/templates/oscar/dashboard/layout.html

@@ -125,6 +125,18 @@
             </div>
         </div>
     </div><!-- /container -->
+
+    {% comment %}
+        This block includes a single pixel from one of Tangent's servers.  We use the referer header to determine which sites are
+        using Oscar.  Since Tangent sponsor Oscar's development, this is an important metric to help justify the investment.  You
+        are, of course, welcome to remove this tracker - but please email the mailing list if you have a production site using Oscar.
+    {% endcomment %}
+    {% block trackerimage %}
+        {% if call_home and not debug %}
+            <img src="//oscar-static.tangentlabs.co.uk/tracker.gif" style="display:none" />
+        {% endif %}
+    {% endblock %}
+
 {% endblock %}
 
 {% block extrascripts %}