Added Skubit OpenIAB support

This adds support for Skubit in-app purchases with Bitcoin. There are two stores supported: the Skubit production store and the Skubit test store (runs on a private blockchain using fake Bitcoins).

I made one small change to SkuDetails to make a method public and I added the Skubit mappings to OpenIabHelper, otherwise core code is untouched.

Skubit is nearly identical to the Google Play IAB, so I just copied the IabHelper class to a new class and made the necessary modifications in the copy.  This means I did not touch core code nor did I create a tightly coupled dependency (since I expect the stores to evolve differently).

I'll make any necessary changes after review.