Added Prophecy

Prophecy is a highly opinionated yet very powerful and flexible PHP object mocking framework. 

It's used as mocking framework in [PHPSpec](https://github.com/phpspec/phpspec) and will eventually be supported by [PHPUnit](https://github.com/sebastianbergmann/phpunit) (see: https://github.com/sebastianbergmann/phpunit/commit/6c21c66567031c863f207fb46b3c1c545d5589d1).