Added Coveralls badge to README

(also fixed linebreak formatting that was causing 'build unknown' and 'coverage unknown' images)

README.rst

@@ -1,4 +1,8 @@
-.. image:: https://travis-ci.org/torchbox/wagtail.png?branch=master   :target: https://travis-ci.org/torchbox/wagtail
+.. image:: https://travis-ci.org/torchbox/wagtail.png?branch=master
+    :target: https://travis-ci.org/torchbox/wagtail
+
+.. image:: https://coveralls.io/repos/torchbox/wagtail/badge.png?branch=master
+    :target: https://coveralls.io/r/torchbox/wagtail?branch=master 
 
 Wagtail CMS
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