Add ability to set class mapping settings as strings

This is important so we can write our own user conversion class and avoid ImportError when trying to import our custom user conversion class on our project settings file:

Now both ways work:

```python
from django_facebook.api import FacebookUserConverter

FACEBOOK_CLASS_MAPPING = {
    'user_conversion': FacebookUserConverter
}
```

or

```python
FACEBOOK_CLASS_MAPPING = {
    'user_conversion': 'django_facebook.api.FacebookUserConverter'
}
```