Supporting Symfony2 bundles without vendor namespace

Using a bundle without a namespace, as suggested in [Official Symfony Best Practices](http://symfony.com/doc/download-best-practices-book/), will result in unexpected behavior.

Expectation as described in the [Feature](https://github.com/chartinger/Symfony2Extension/blob/master/features/describing_a_class.feature#L30) file:
```
| Class | Specification |
| Scenario1/Bundle/DemoBundle/Model/User | src/Scenario1/Bundle/DemoBundle/Spec/Model/UserSpec.php |
| Acme/DemoBundle/Model/User | src/Acme/DemoBundle/Spec/Model/UserSpec.php |
| DemoBundle/Model/User | src/DemoBundle/Spec/Model/UserSpec.php |
```

To do this pull request, it updated Behat related files to support version 3, using helper files from [PHPSpec](https://github.com/phpspec/phpspec)

I am not sure about the expected behavior when files are without a bundle (eg in [this spec](https://github.com/phpspec/Symfony2Extension/blob/master/spec/PhpSpec/Symfony2Extension/Locator/PSR0ResourceSpec.php#L119)), i mimicked the [current implementation](https://github.com/phpspec/Symfony2Extension/blob/master/src/PhpSpec/Symfony2Extension/Locator/PSR0Resource.php#L127) to satisfy the specs although it feels a bit odd to me.