Added discriminated events functionality.

issue #14

Hey,

This is a simple implementation of this feature. Let me now what do you think. Basically listener subscribes to all FOSUserEvents and when any of those events is fired it dispatches a new event by prefixing the user system name to the original event name. 

I did not defined constants for the new event names because I wasn't sure what would be the best way to do it. What do you think should global constants be defined with this new event names?