README: add coveralls badge

Author: Will Roberts

README.rst

@@ -6,6 +6,10 @@
     :target: https://travis-ci.org/wroberts/pytimeparse
     :alt: Travis CI build status
 
+.. image:: https://coveralls.io/repos/wroberts/pytimeparse/badge.svg
+    :target: https://coveralls.io/r/wroberts/pytimeparse
+    :alt: Test code coverage
+
 .. image:: https://pypip.in/version/pytimeparse/badge.svg
     :target: https://pypi.python.org/pypi/pytimeparse/
     :alt: Latest Version