Adds API to display details of a member.

Ajoute la possibilité de consulter un membre donné via l'API.

Exemple d'URL :

```
http://localhost:8000/api/membres/1/
```

Exemple de réponse :

```json
{
    "id": 1, 
    "username": "admin", 
    "show_email": false, 
    "email": "", 
    "is_active": true, 
    "site": "", 
    "avatar_url": null, 
    "biography": "", 
    "sign": "", 
    "email_for_answer": false, 
    "last_visit": "2014-12-04T00:22:06.714", 
    "date_joined": "2014-12-02T21:43:52.887"
}
```