Tweaking specs for managedObjectUniquingPredicate:

This relates to github/Mantle#117

Yo dawg, I heard you like Pull Requests…

I felt that ultimately, the _should fetch a unique parent managed object using numberString to maintain uniqueness_ spec was mostly duplication and checking all children and properties didn't add much to validate uniqueness. I've thus replaced it with a smaller spec in the `+managedObjectFromModel:insertingIntoContext:error:` clause. (See d2094446dfaa31ac89e9a41e49d9bae169602d81)

I hope this doesn't get too confusing, I fixed some whitespace and merged in `master` for good measure.