Add coveralls

Coveralls.io is a nice service to publicly track coverage percent.

Right now the badge shows around ~ 50-60% coverage, which is good.

[![Coverage Status](https://coveralls.io/repos/coagulant/sorl-thumbnail/badge.png?branch=coveralls)](https://coveralls.io/r/coagulant/sorl-thumbnail?branch=coveralls)

I think it's good to have coverage dynamics visible to everybody and be able to track coverage ups and downs in pull requests.

To enable coveralls for origin repo, you need to login via github on coveralls.io and enable mariocesar/sorl-thumbnail repo