More strict type checking, and some docblock and style improvements.
Major bug fix in ComposerJson and ComposerJsonTargetDir:
Local variable $data was not set in ComposerJson[..]::writeToAdapter().
Serious bugfix in ComposerJson:
$path_prefix must end with '/'.
Bugfix in ComposerJson: Allow empty paths.
Add fixtures libraries for ComposerJson and ComposerJsonTargetDir.
Add ComposerJsonTest class.