different request and response serializers  

Successor of #19
---

It is now possible to use different serializers for the request and the response. 
I introduced *request_serializer_class* and *response_serializer_class* attributes. If they are not found, it still falls back to *serializer_class*

 ```python
class PlacesPrefetchRequest(APIView):
    """
    Prefetch places
    """
    
    request_serializer_class = serializers.SimplePlacesSerializer
    response_serializer_class = serializers.PlacesSerializer
    
    def post(self, request, *args, **kwargs):
        [...]
 ```