v3.0.2 Bugfix Release

#70: add changelog and update version tag in source
#73: fix some code smells and add a contribution manual
#75: update phpunit to 8.x (and fix all the deprecation issues)
#78: clean up code fragments after adding type hints to almost everything - PHPTAL::addPreFilter no longer accepts FilterInterface but requires an instance of PreFilter - PHPTAL::setPhpCodeDestination now returns void instead of self - SourceInterface::getLastModifiedTime now returns float instead of int