Add Circle CI badge to README
.. image:: https://img.shields.io/travis/cherrypy/cherrypy/master.svg?label=Linux%20build%20%40%20Travis%20CI
:target: http://travis-ci.org/cherrypy/cherrypy
+.. image:: https://circleci.com/gh/cherrypy/cherrypy/tree/master.svg?style=svg
+ :target: https://circleci.com/gh/cherrypy/cherrypy/tree/master
+
.. image:: https://img.shields.io/appveyor/ci/CherryPy/cherrypy/master.svg?label=Windows%20build%20%40%20Appveyor
:target: https://ci.appveyor.com/project/CherryPy/cherrypy/branch/master
