Sign out inactive sesions in embedded apps

This way, when a user logs out from BookingSync, but doesn't close
the browser window, after a period of time defined in
BookingSync::Engine.sign_out_after (default 10 minutes), the session
will become invalid and the user will have to log in again.
Merge pull request #5 from BookingSync/sign-out-after-inactivity

Sign out inactive sesions in embedded apps