use openssl_random_pseudo_bytes to generate random bytes

since the uniqueness of uuid(4) depends highly on the generated bytes, we should use the strongest implementation possible. In addition to openssl_random_pseudo_bytes we could think about trying /dev/urandom before using mt_rand as last resort. What do you think?

I'm quite sure I didn't break something but I'm not 100% sure since I've not able to run the unittests. Would be great if you could take a look at it.

```bash
colibri:uuid (master %)$ phpunit
PHPUnit 3.7.22 by Sebastian Bergmann.

The Xdebug extension is not loaded. No code coverage will be generated.


PHP Fatal error:  Class 'Doctrine\Tests\DBAL\Mocks\MockPlatform' not found in /Users/Daniel/dev/uuid/tests/Rhumsaa/Uuid/Doctrine/UuidTypeTest.php on line 27

Fatal error: Class 'Doctrine\Tests\DBAL\Mocks\MockPlatform' not found in /Users/Daniel/dev/uuid/tests/Rhumsaa/Uuid/Doctrine/UuidTypeTest.php on line 27
```