Add coveralls code coverage button

[alexanderdean]

https://coveralls.io/docs/python

[fblundun]

Reassiging to Alex... This is the page I used to let Coveralls access my repositories: #https://coveralls.io/authorize/github
But I wasn't able to add the Python Tracker repo, I think because I didn't create it.

[alexanderdean]

I have enabled it, reassigning to Fred...

[alexanderdean]

Done, closing.