Initial draft of accounts manager and related protocols

The accounts manager also acts as a data source
for view controllers. It also can optionally
persist accounts to and from disk (user defaults).

Also:

Allow accounts to clean up themselves and their
authenticator discard its credential. The account
also acts as a data source for views.
Harden account button's data source handling
Initial draft of accounts view controller

The view controller observes its data source to
respond to state changes. It is mostly a
conventional view controller (UIAppearance,
customizable attributes, etc.) with an account
view and action buttons that only shows as needed
and update themselves depending on state.

By default, the view controller automatically
prompts for the nation slug if there is no active
account.
Integrate account manager and view controller into sample app

Rewrite integration in app delegate to mostly
consist of implementing delegate methods and lazy
initialization with minor configuration.

Expand 'ready' aspect in people view controller.