#18 configurable time-out of sessions

[Daniel Santamaria]

currently there is no way that a session times out.
ideally, the admin should be able to configure a
period, after which the session will become idle. warn
the user of this time, perhaps in the header where it
says "logged in as" using ajax, so they don't lose all
the data they are working on while idle inputting
grades for a long time.