adds/improves logging (includes PR #13)

Namespace usage for the Psr\Log library requires PHP >= 5.3. This shouldn't be an issue as Composer itself requires PHP 5.3.2+ to run (https://getcomposer.org/doc/00-intro.md#system-requirements).

PHP 5.2 has been dead for quite a while and PHP 5.3 has only been receiving security updates for the past year (http://php.net/archive/2013.php#id2013-07-11-1).