added log in form to new.html.erb in views/sessions
created sessions controller to handle login/logout
assigned session id to @user.id upon signup so user logged in on successful signup
created sessions routes
created login form
created current_user method and assigned it as a helper method so it can be used in the views
created header based on whether user logged in or not
added validations to user model, moved signup link from index to application.html.erb