uuid provider

A uuid provider with unit tests.
It is not registered as default provider.

Details:
* name based UUID (md5) since it is fast and predictable
* does generate same sequence for a given seed (`mt_srand()`)

Use:
```php
$faker = new Faker\Generator();
$faker->addProvider(new \Faker\Provider\Uuid($faker));
$faker->uuid; // "7e57d004-2b97-0e7a-b45f-5387367791cd"
```