Fix localized date

This PR fixes the localized date and introduces a minor BC break (easy to fix):

* instead of `if column.formOptions.datetime_format is defined`
* it checks for `if column.formOptions.format is defined`

The enhancement is that it uses `format` option (which is also used by symfony2's base DATE field) as a pattern for translation.

Now, you can translate your date anyhow you want. For example, you can output it in `MMMM YYYY` format:
`December 2013` (not displaying the days).

Another good thing is that you don't have to do:

```yaml
params:
    fields:
        mydate:
            addFormOptions:
                format: "Y-m-d"
            date_format: "Y-m-d"
```

Instead simply set the format option:

```yaml
params:
    fields:
        mydate:
            addFormOptions:
                format: "Y-m-d"
```