New authentication.TokenBackend Django's backend

To be used at settings like this:

```python
AUTHENTICATION_BACKENDS = (
    'django.contrib.auth.backends.ModelBackend',
    'rest_framework.authentication.TokenBackend',
)
```

and on views like this:
```python
user = authenticate(token=token_key)
if user is not None:
    login(request, user)
```