Session Lister: Lists all concurrent sessions off a site

This class is meant to lists all file based concurrent sessions on a site.
So you can have info on all "logged" users of a site. (who's online...)

It needs read access to session files temporary directory.

V1.1 : returns the creation date, the last modif date, but also the age of the session (as very old sessions files
are sometimes present on the server). if the session is older than 4 hours for example, you can choose to ignore it.
So you can for example list all users logged within the last 4 hours.