YAML file configuration and Command to generate YML from an entity

Hi,

After few talks about it, here is a pull request to add yml file configuration and a command to generate theses file from an entity.

YML file configuration
-------------------------------
- Like doctrine, YML file configuration for an entity must be placed in a special directory and have a special name MyBundle/Resources/config/grid/MyEntity.myGroup.grid.yml
- The YML file structure is the following:
````
# Resources/config/grid/MyEntity.myGroup.grid.yml
namespace\of\MyEntity
    Source:
        columns: 
            id: ~
            myColumn1: ~
        groupBy: ~
        filterable: ~
    Columns:
        id:
            filterable: true
            visible: true
            id: id
            title: columnTitle
        myColumn1: ....
    ...
````
- Annotation and YML Driver are both loaded in services.xml

YML Generation command
-------------------------------
- The command will allow you to generate a YML config file for an entity and a group:

```
php app/console apydatagrid:generate:grid --entity="MyBundle:MyEntity" --group="default"
```

- The file will be generated and written in src/MyBundle/Resources/config/grid/MyEntity.default.grid.yml.
- The generation of the file is based on a skeleton template stored in APYDataGrid/Resourses/skeleton/grid/grid.yml.twig.

Feel free to make any comments on this PR !