Set field to string

[seawolf42]

It would be very convenient if a user could use the field like this:

class MyModel(models.Model):
    lookup = LookupTableItemField(table_ref='abc')

instance = MyModel.objects.get(pk=some_pk)
instance.lookup = 'the name of an option'
instance.save()

This requires marshaling the field to find the right LookupTableItem for that value. If no such item exists, this should be an error.

[seawolf42]

possibly relates to #27