Add a helper to user generators as PHPUnit data providers

Using this, you can easily use generators as providers like it is done in the tests :

```
    function provider($name, $n = 10) {
        return DataProvider::provider(array(
            Generator::strings(),
            Generator::ints(),
            Generator::booleans()
        ), $n);
    }

    /**
     * @dataProvider provider
     */
    function testDataProvider($s, $i, $b) {
        // this test is only supposed to prove that we have a valid provider
        $this->assertTrue(is_string($s));
        $this->assertTrue(is_int($i));
        $this->assertTrue(is_bool($b));
    }
```