It seems that the coveralls badge is available in README, but we don't integrate this code coverage service in this repository.
To integrate the coveralls service, I think we have to install the php-coveralls package and generate the coveralls.xml via phpunit running unit tests.
Then upload coveralls.xml report via php-coveralls package to specific code coverage page for this repository.
So, I want to better separate current test components. I have started to implement a new strategy for future tests to follow, as seen in 920b6fb and e744669. This strategy is: a single test per exception class. This issue was opened to remind me to finish by the 1.0.0 milestone