Added entropy sources + unit test

I added some other entropy sources for PHP: getmypid(), $_ENV, $_SERVER, posix_times(), zend_thread_id().
I provided a unit test class using PHPUnit, you can run the test under the tests folder using the command:
$ phpunit SrandTest.php
I also separated the implementation in two files, one for the function secure_random_bytes() and another for the demo.