Adds a few methods to application controller

- current_user
- sign_in
- authenticate
- signed_in?
Adds before filter for authenticate.

- to cards & decks controllers
Adds find_deck method to decks controller

- Also makes controller use current_user
Updates cards controller to use current_user
Adds homes controller, views and route
Adds sessions for logging in
Reorganizes routes file
Signs user in after create

- So that it does not break when redirecting to decks_path
because they are not signed in
Puts the new deck link in a div