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.
I don't particularly like this lopsidedness either. Do you think there is a better way to handle this?