Update for iOS 7 style receipts

There are [previous](https://github.com/nomad/venice/pull/18) PR's for iOS 7 receipts, and some questions still remain. I personally have two projects that require a quality implementation so I'd love to contribute whatever is lacking. 

I'd like some insight on backwards compatibility. With these updates would Venice push to a significant version bump? 

I don't think spec coverage is near complete in this implementation but the baseline is there as is fixes for specs that were broken immediately when forked.

This PR includes @coneybeare's commits from February.