Refactor determined_from_responses methods

Now we're getting more and more meta mappings, I've found the need for there to be more and more `*_determined_from_responses` style methods that do pretty much the same thing in `ResponseSet`.

To DRY things up a bit, I've used `method_missing` to generate the functions, so all you need to do now to add a mapping is edit the `map` method in `Survey`