Add support for simplified YAML/XML drivers

I created this PR to add support for simplified YAML/XML drivers ([more information][1]), by providing the ``simple_yml`` and ``simple_xml`` types.

```
The Symfony project sponsored a driver that simplifies usage of the YAML Driver. The changes between the original driver are:

* File Extension is .orm.yml
* Filenames are shortened, “MyProject\Entities\User” will become User.orm.yml
* You can add a global file and add multiple entities in this file.
```

I think this PR should be merged in the 1.x branch, as BC is maintained (it provides an extra feature). I hope I 'm doing this right.

[1]: http://docs.doctrine-project.org/en/latest/reference/yaml-mapping.html#simplified-yaml-driver