First draft for real path mapping in FOSUserRegistrationHandler

This is a idea in order to enhance the pathresponse. With that you could create custom mapping in configuration for your users. (If they are based on FOSUSerbundle).

My usecase is that facebook provides more data then currently is mapped. And we have already the possibility to map those values in the response object. But the mapping from this responseobject to the user was hardcoded and only allowed to set the username and the email. With this PR you could be able to  map firstName, lastName etc. 

I would like to get feedback on this. But have a look at the impact with the Response->getNickname() <-> username mapping. Maybe we would need to change the default mapping for some resources owners.

Any feedback on this?