Include language_family in language objects (incl results)

Fixes #164 

language_family is now part of the LanguageSerializer. Language JSON objects will look like this:
```json
{
  "id": 4, 
  "iso_code": 4, 
  "name": "Chenoua",
  "language_family": {
    "id": 1, 
    "language_count": 112, 
    "level": 1, 
    "name": "Afro-Asiatic", 
    "parent": null
  }
}
```

If we need to add subfamily or subsubfamily, those could be done the same way.