Message set importer

Passing a ``StringIO`` into a delay call is a pretty weird thing to do. I'm a bit surprised it works at all (I suspect it only works when DEBUG=True and the delay call is a dummy). Either just pass the string (``csv_data = ...read()``) or store the file temporarily on disk somewhere (see https://github.com/praekelt/vumi-go/blob/develop/go/contacts/utils.py#L9-L16).