Csrf Protection

CSRF Protection for Backbone.js and Rails.


I don't use `toJSON` method.
In Backbone.sync, add `X-CSRF-Token` Header.


* Ref
http://blog.softr.li/post/43146401263/finally-correctly-dealing-with-rails-csrf-protection
https://github.com/codebrew/backbone-rails/blob/master/vendor/assets/javascripts/backbone_rails_sync.js