Add coverall badge in README

Author: obenjiro

README.rst

@@ -1,7 +1,7 @@
 Python version of SimplePEG
 --------
-.. image:: https://travis-ci.org/SimplePEG/Python.svg?branch=master
-    :target: https://travis-ci.org/SimplePEG/Python
+.. image:: https://travis-ci.org/SimplePEG/Python.svg?branch=master :target: https://travis-ci.org/SimplePEG/Python
+.. image:: https://coveralls.io/repos/github/SimplePEG/Python/badge.svg?branch=master :target: https://coveralls.io/github/SimplePEG/Python?branch=master
 
 To use, simply do::