Add initial sample app

This changeset's commits are structured around groups of related files. The goal of the sample app (NB Sample) is to be an modest example of how to write an app on top of the NBClient library using modern iOS conventions and only iOS interfaces and stock UIKit classes. This changeset accounts for all the app's required features, with the major exception of the authentication flow. This changeset also does away with the NBClientCatalog project, since the app just represents a collection of people.

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