Essential User stories
  As a user I should be able to sign up for Shelf
  As a user I should be able to log in to Shelf
  As a user I should be able to add a new reading to Shelf
  As a user I should be able to view all my readings or a single reading on Shelf
  As a user I should be able to update a reading on Shelf
  As a user I should be able to delete a reading on Shelf

Screens and user flow
  Screen for signing up for Shelf
    user flow
      user enters email / password and clicks submit --> returns them to home screen with message saying signup successful
      user does not enter an email or password --> returns error message to supply username and password
      user does not enter email and clicks submit --> returns error message to supply email
      user does not enter password and clicks submit --> returns error message to supply password
      user enters email that is taken --> return error message that email is taken
  Screen for logging into Shelf
    user flow
      user enters email / password and clicks submit --> redirects user to authenticated landing page showing users readings
      user does not enter an email or password --> returns error message to supply username and password
      user does not enter email and clicks submit --> returns error message to supply email
      user does not enter password and clicks submit --> returns error message to supply password
      user enters invalid email or password --> return error message that login failed and to check credentials
  Screen for adding new readings
    user flow
      On Books page user clicks add Book button. In the popup user searches for a title and makes selection. User confirms book selection to be added. User is redirected to default Books screen.
  Screen for updating readings
    user flow
      On Books screen user selects a book. User selects Edit option and is able to edit any of the editable fields. User clicks save when finished and book will be updated. User is redirected to default Books screen.
  Screen for deleting readings
    user flow
      On Books screen user selects a book. User selects Delete option and needs to confirm deletion and book will be deleted. User is redirected to default Books screenUser model