Add initial accounts widget

The account model builds upon the client and authenticator objects. It provides a simpler interface than manually managing a client and an authenticator. It can become the data source for views including the account button.

The account manager builds on top of the account model, and unlike the lower level classes, it relies on delegation to integrate with other classes, ie the app delegate. It provides a simpler interface than manually managing one or more accounts. It can become the data source for the accounts view controller.

Adds implicit authentication support, mostly implemented by the authenticator. Also adds the initial (untested) support for view customization. Also introduces minor UIKit additions.

Includes integration with sample app, with the essential comments. Includes updating the people list to operate based on sign-in state.