Install OAuth2 dependencies

The omniauth gem handles most of the OAuth dance for us, and the omniauth-oauth2 gem can be used to implement a custom OAuth2 strategy.
Implement custom OAuth2 strategy

This strategy tells omniauth how to reach the OAuth provider, and how to retrieve additional information.
Add initializer for Chandelier

Right now, the only thing this initializer does is to configure omniauth.
Require strategy

The omniauth strategy for Chandelier needs to be required inside the initializer file.
Generate SessionsController

This controller handles everything related to user sessions.
Add login check to SessionsHelper

The helper contains a method can be used as a before_filter in controllers to allow only authenticated users to call an action.
Fix callback path in omniauth strategy

The path in omniauth strategy example does not match our callback path. Therefore, we changed it.
Implement SessionsController

The SessionsController handles user sessions, and has actions to create and destroy those. Also, it has a callback method for OAuth in case an authentication failed.
Add methods to SessionsHelper

The two methods provide the some interface as devise, and can be used to implement a similar behavior.
Implement single sign off

A single sign off mechanism has been implemented based in cookies.
Merge branch 'feature/oauth-integration' into develop