Adjust coverage to only include oscar package

.coveragerc

+[run]
+source = oscar

Makefile

 	cd docs && make html
 
 coverage:
-	./runtests.py --with-coverage --cover-package=oscar --cover-html --cover-html-dir=htmlcov --with-xunit
+	coverage run ./runtests.py
 	coverage xml -i
 
 # We probably should use upgrade instead of install here but we have a conflict

README.rst

 .. image:: https://secure.travis-ci.org/tangentlabs/django-oscar.png
     :target: http://travis-ci.org/#!/tangentlabs/django-oscar
 
+.. image:: https://coveralls.io/repos/tangentlabs/django-oscar/badge.png?branch=master
+    :alt: Coverage
+    :target: https://coveralls.io/r/tangentlabs/django-oscar
+
 .. _`Official homepage`: http://oscarcommerce.com
 .. _`Sandbox site`: http://latest.oscarcommerce.com
 .. _`Documentation`: http://django-oscar.readthedocs.org/en/latest/