WPAccount handles signout and notifications

- removeDefaultWordPressAccount handles credential removal, user
  defaults reset, and uses the proper notification for the app

- PushManager -> NotificationsManager

- Category for the XMLRPC API usage, so it's easy to move it out later
  to the API pod
Use WPAccount system throughout instead of WordPressComApi.

- Using isWpComAuthenticated in place of hasCredentials on
  WordPressComApi

- Notifications actions decoupling from WordPressComApi
  - Fetching/Saving
  - Checking for unseen notifications