This class is intended to extend the $_SESSION magic variable by providing an API hook
to plug in other values.
Primarily this is intended to provide a way of supplying "logged in user" details without touching the session
(which can cause problems when accessed server side).
If a value is present in the session then that value is returned, otherwise a plugin hook 'session:get', '$var' is called,
where $var is the variable being requested.
Setting values will store variables in the session in the normal way.
LIMITATIONS: You can not access multidimensional arrays
This is EXPERIMENTAL.