Add "Surprise Me!" product option to the iOS store

Adds a "Surprise Me!" option to the iOS store, which selects a free product from the available products and configures a random size and colour. The user will not know what the mystery product is until the Printfection shipping confirmation is sent! 

If the ?includeMonkeys parameter is set in Webservice.cs, the monkey is also an possible surprise product. In fact, any free product (price < 0.01) returned by the web service is eligible.

Changes:

* allShirts/@2x.png: surprise product graphics

* 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

* XamStoreStyleKit.cs: playing with PaintCode 2.0