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.
It's been a while since I looked at it, but I think the `RegisterHITType` call needs the QRs to be flattened into the map with the rest of the fields, like `title` and `description`.

Which is fine, I think the reason I did that was because QRs are built separately using `biomass.builder.qualification-requirements/build` and as an API you would end up doing a `merge` in your code like:
```
(defn hit-type
  []
  (biomass.hits/register-hit-type
   (merge
    {:title "Transcribe Receipt Details"
     :description "Identify the 1) Biller,  2) Tax"
     :reward-amount 0.10
     :reward-currency "USD"
     :assignment-duration 180} (qualification-for-receipt))))
```
I had also changed the builder to do multiple QRs (afterwards), so in which case you would only merge once. I think it makes sense to do it in the `params` map, yes.