Add PHP-Mock (#643)

PHP-Mock is a library to mock built-in PHP functions (e.g. time()).

See also: https://github.com/php-mock/php-mock

README.md

 * [Peridot](https://github.com/peridot-php/peridot) - An event driven test framework.
 * [Phake](https://github.com/mlively/Phake) - Another mock object library for testing.
 * [Pho](https://github.com/danielstjules/pho) - Another behaviour driven development testing framework.
+* [PHP-Mock](https://github.com/php-mock/php-mock) - A mock library for built-in PHP functions (e.g. time()).
 * [PHPSpec](https://github.com/phpspec/phpspec) - A design by specification unit testing library.
 * [PHPT](https://qa.php.net/write-test.php) - A test tool used by PHP itself.
 * [PHPUnit](https://github.com/sebastianbergmann/phpunit) - A unit testing framework.