Add basic support for linked data.

Basic support means allow to generate names with `@` sign, by passing `ld` or `linked_data` argument into field declaration, like:

``` python
url = fields.CharField(linked_data=True)
```

or

``` python
url = fields.CharField(ld=True)
```

Field should be considered as linked either if `ld` or `linked_data` is `True`.