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
don't check coverage on external node_modules
Fix express to 4.2.0 due ot build break
fix variable names, cleaner filter
Merge pull request #906 from stuartf/coverageIgnoreExternal

don't check coverage on external node_modules
Merge pull request #907 from mrvisser/express-4.2.0

Fix express to 4.2.0 due to build break
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
Improve example CAS logoutUrl on admin UI
Bump oae-rest back to latest
Merge with master