Div node management

Some other things in this PL:

* Added PHPUnit to start testing the library and avoid regresison. I usually keep PHPUnit inside the project so all the contributors can use the same version.
* Implemented the `div` functionality
* Started to write unit test and to use `PHP_EOL` instead of '\n'

To launch tests (from the root directory of the project):

```bash
# vendor/bin/phpunit
```