Initial support for receipts Issue #427

review? @rpl @mykmelez

I created an [app](https://github.com/nickdesaulniers/simulatorTestReceipt) that uses our [receiptverifier](https://github.com/mozilla/receiptverifier) library (and found some bugs in it along the way).  Please see the [README](https://github.com/nickdesaulniers/simulatorTestReceipt/blob/master/README.md) for instructions on how to test.