adds Psr\Log as a dependency
adds Clever::setLogger()
replaces error_log() with $logger->notice()
Merge branch 'refs/heads/phpunit' into logger
adds use statement
namespace usage for the Psr\Log library requires >= 5.3

PHP 5.2 has been dead for quite a while and PHP 5.3 has only been receiving security updates for the past year.

see: http://php.net/archive/2013.php#id2013-07-11-1
removes "Tested on ..." comment

lib successfully tested on PHP 5.3.*, 5.4.*, 5.5.*
Merge pull request #14 from capdig/logger

adds/improves logging (includes PR #13)