Add support for external logout

Each OAE authentication strategy can now expose a `logout` function
that will called when a user that authenticated with that strategy logs
out.

This has currently only been implemented for CAS
External authentication logout

 * Removed ctx from `strategy.logout`
 * Cleaned up CAS tests
External logout follow-up

 * Redirecting to `/` for strategies with no logout function
 * Asserting regular logout redirect in tests
Use new `casRedirect` api
Specifying full CAS logout url
Fixing oae-rest to authentication-endpoints branch
Merge pull request #876 from simong/cas-logout

Add support for external authentication logout