Major session back-end overhaul. Use HttpFoundation for sessions.

+- Migrated entire session back-end to Symfony HttpFoundation Session.
The `native` driver should now be used in place of the `cookie` driver.
All other drivers are available and work the same. New sessions will
not be backwards compatible after updating.
+- Renamed `Session::getToken` to `Session::token`.
Added getToken for BC.
Fix bug in session store.