[add] unit test for phpDocumentor\Plugin\Core\ServiceProvider::register()
Merge branch 'develop' of github.com:madmanmax/phpDocumentor2 into madmanmax-develop
Refactor ServiceProvider and test for Clean Code

Thanks to the contribution of @madmanmax of a unit test for the Service
Provider of the Core Plugin I had the chance to refactor both to match
the Clean Code standards and add a but of readability.