Update AuthenticationListener.php

When using the security switch user feature (from an admin to a discriminant user), it should listen and set the proper discriminator.
Add SecuritySwitchUser test to AuthenticationListenerTest
Merge branch 'patch-1' of https://github.com/redstar504/PUGXMultiUserBundle
Merge pull request #27 from redstar504/patch-1

SecuritySwitchUser Listener