Add code coverage

[vv12131415]

I don't see code coverage for the project.
It is very important, since this project is very popular

[rodrigoprimo]

Code coverage tracking has been implemented in the new repo. PR PHPCSStandards/PHP_CodeSniffer#144 added Coveralls integration, and coverage reports are available at https://coveralls.io/github/PHPCSStandards/PHP_CodeSniffer.

I believe this issue can be closed.