Attachments related bugfixes 

This includes fixes for:
- #814 Se pueden crear attachments con al API asociados a cualquier objecto
- #813 Se pueden crear attachments con el API sin adjunto

The #813 implies disable not partial updates request (PUT), because django (and almost all frameworks) does not have support for put multipart requests. But now, the attachment serializer has attached_file field as required. With that conditions, put request are automatically not supported, because you never can upload file... For it, I have disabled not partial updates explicitly, raising method not supported (405).