Bugfix:

 * \DateTime::__construct is exception happy and cannot parse a timestamp
 * use \DateTime::setTimestamp() for numbers (this is an assumption)
 * makes the tests run (apparently something changed somewhere in PHP 5.4.11)
Throw \LogicException when we fail to create a \DateTime object.
Merge pull request #4 from till/bugfix/helper

Bugfix/helper, fix #3