add phpcs as dev-depency

like #18 i'd add phpcs as dev-depency as well. makes it easier to control the tools (and their versions) one should use to verify that everything works as expected.

    % vendor/bin/phpcs --standard=PSR src tests
    ERROR: the "PSR" coding standard is not installed. The installed coding standards are MySource, PEAR, PHPCS, PSR1, PSR2, Squiz and Zend

is that because i'm missing the `PSR` rules in the default installation of `phpcs` or do we have to name another standard to be executed w/ that command?