Build QualificationRequirements

I'm raising this pull request just so that you can track progress.

It currently supports specifying multiple QRs and passing them along when you're registering a HIT type. It probably needs a few tests before it's merged. I'm not familiar with Midje, so it might take a while until I get around writing those.

I feel this is potentially a bit cluttered in its implementation. See if it makes sense. Meanwhile, I'm also working on handling exceptions and response codes from the MTurk API.
Any reason you can't just call the ref sandboxed?

That way you can check it with @sandboxed?