Add discriminated events dispatcher

                     
|Q            |A    |
|---          |---  |
|Bug Fix?     |n    |
|New Feature? |y    |
|BC Breaks?   |n    |
|Deprecations?|n    |
|Tests Pass?  |n    |
|Fixed Tickets|GH-14|
|License      |MIT  |
|Doc PR       |     |
                     

The EventDiscriminator re-dispatches FOSUserBundle events for the current usersystem. This idea was originally proposed by @dramos. But this implementation uses less magic.

**Todo**

* [ ] Add tests
* [ ] Update documentation