* allShirts.png:
* allShirts@2x.png: surprise product graphics

* XamStoreStyleKit.cs: playing with PaintCode 2.0

* Product.cs: add SurpriseProduct subclass and helpers

* BasketViewController.cs: mask SurpriseProducts in basket view

* ProductListViewController.cs: add mystery product option to product
  list

* XamarinStore.iOS.csproj: add mystery product functionality to store