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 could turn it into a `doseq` or a `for` at best – but that would be still structurally similar, more idiomatic I suppose. The only reason it is this way is because `request-params-build` requires a number as the params sent across are numbered based on how many you pass in. 

Other options: increment an atom while reducing over the qualifications.