Added support for option "format" on "date" mapping type

Added posibility to specify the date format:

```yaml
mappings:
    username: ~
    profile:
        type: nested
        properties:
            name: { boost: 2.0 }
            gender: ~
            birthday: { type: date, format: "yyyy-MM-dd" }
```

http://www.elasticsearch.org/guide/reference/mapping/date-format/