Placed sign out for an account in WPAccount.

- Renamed PushManaged to NotificationsManager for clarity

- Bringing a lot of the WordPressComApi functionality into WPAccount as
  a category

- Notifications API calls will go into a category off
  NotificationsManager

- Goal is to have app-agnostic WordPressComApi code in a pod
Decouple app-specific functionality for Accounts

- WordPressComApi accepts a username/password, and via blocks returns a
  token

- Sign in via WPAccount

- Remove older approach to networking classes with delegation