Added Linked-In Login feature which allow user to login into the

applications using user's Linked-In Account. Used devise's
        omniauth. Basic core programming for login added.

    ============= Added (6)

        /app/controllers/users/omniauth_callbacks_controller.rb
        /app/models/authentication.rb
        /db/migrate/20130126222824_create_authentications.rb
        /lib/mentor_mentee_helpers/auth/external_provider.rb
        /test/fixtures/authentications.yml
        /test/unit/authentication_test.rb

    ============= Updated (12)
        /Gemfile
            - Added gems for liinked omniauth
                gem 'omniauth'
                gem 'omniauth-linkedin'

        /app/models/user.rb
            - Added relation to Authentication model
                has_many :authentications, dependent: :delete_all
            - Added Devise's omniauthable feature
            - Added methods to support omniauth
                apply_omniauth
                connected_to_linkedin

        /app/views/users/sessions/new.html.haml
            - Added link to login via Linked-In

        /config/initializers/devise.rb
            - Added OmniAuth Linked-In configurations
                config.omniauth :linkedin, Settings.omniauth.linkedin.key, Settings.omniauth.linkedin.secret, omniauth_options

        /config/locales/en.yml
            - Added external login messages
                external_login{disconnect, errors}

        /config/routes.rb
            - Configured devise for omniauth_callbacks
                devise_for :users, controllers: { omniauth_callbacks: "users/omniauth_callbacks" }
            - Added devise scope
                get '/users/auth/:provider/disconnect' => "users/omniauth_callbacks#disconnect", as: :auth_disconnect

        /config/settings.yml
            - Added omniauth settings for linkedin

        /db/schema.rb
            - Added following table:
               a) authentications