Added .yml configuration builder

From it will be possible to configure datasource fields in .yml file located in ``Resourcec/config/datasource/`` folder. 
Example configuration: 
```yml
#src/FSi/Bundle/DemoBundle/Resources/config/datasource/news.yml
fields:
  title:
    type: text
    comparison: like
    options:
      form_options:
        label: admin.news.list.title
  created_at:
    type: date
    comparison: between
    options:
      field: createdAt
      sortable: true
      form_from_options:
        widget: single_text
        label: admin.news.list.created_at_from
      form_to_options:
        widget: single_text
        label: admin.news.list.created_at_to
  visible:
    type: boolean
    comparison: eq
    options:
      sortable: false
      form_options:
        label: admin.news.list.visible
  creator_email:
    type: text
    comparison: like
    options:
      field: creatorEmail
      sortable: true
      form_options:
        label: admin.news.list.creator_email
```

Configuration file name must be equal to datasource name.