Merge branch 'add_sessions' of github.com:oliverswitzer/leftovers-sinatra-angular into add_sessions

Conflicts:
	Gemfile
	app.rb
	config.ru
	public/app/css/app.css
	public/app/js/responsive.js
Fix merging
Merge branch 'add_sessions' of github.com:oliverswitzer/leftovers-sinatra-angular into add_sessions
Add basic login/logout flow with logout button in pickups.html, logout route in app.rb and logged_in helper method in app.rb