Now logging 'switched user name' when AuthenticationSwitchUserEvent

Mr. Taylor,
  The subject says it all.  I have a project that will be using the switch user feature of Spring Security.  Your plugin did not log the important detail of this event, so I added it and hence this pull request.  Please see summary of change below.

I would be most happy if you decide to merge this change into your plugin's master.

Best Regards,
  Ressom

Summary of change:
-------
 - Updated domain object
 - Updated main logger object
 - Removed deprecated usages in plugin definition
 - Added unit test and updated existing tests
 - Updated README.md to reflect new features.