Add a Tab Bar navigation controller for the popular a user stories

This adds a tab bar for navigating between Popular Tales and User Tales.

I also included the Login view controller into the tab bar for now. @snekkalapudi, @rvharsha we need to figure out how we want to show the login view if the user isn't currently logged in. Most app I think create a custom container view controller that swaps out the view controllers. The easiest approach is to just show a modal though.