add auth helper method
add signin/signout feature spec

* rename requests to features (it become default from capybara2.0)
 * refs: http://alindeman.github.com/2012/11/11/rspec-rails-and-capybara-2.0-what-you-need-to-know.html
Avoid to define helper method on top-level context

refs: https://github.com/fastladder/fastladder/pull/48#discussion_r3403790
Merge branch 'banyan-add-auth-feature-spec'