Fix log cleanup not working

Problem was that the cron handler was only defined in is_admin(), and it was using 'expire' instead of 'expire_redirect' and 'expire_404'

Fixes #18
Update README.md
Display error if accessing an invalid group

Fixes #23
Update readme for 2.3.11
Merge branch 'master' of https://github.com/johngodley/redirection