add EventDiscriminator to re-dispatch events for the active user-system

This idea was originally proposed by GitHub user dramos
But this implementation uses less magic
Merge pull request #39 from rollerworks/events

This PR was merged into rollerworks:master branch.

Discussion
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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

Commits
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959a8f2f62da5fa507bdf56d90157004e0c18479 add EventDiscriminator to re-dispatch events for the active user-system

This idea was originally proposed by GitHub user dramos
But this implementation uses less magic sstok