Update bundle after FOSUserBundle changes

Hi Lio, 

This is a suggestion to update the bundle according to the recent changes made in `FOSUserBundle` and fix [issue 9](https://github.com/PUGX/PUGXMultiUserBundle/issues/9).

As you can see in the readme file, this involves binding the form from the PUGX controller to display the validated form which I don't like very much (as it is already done in the registration controller of FOSUserBundle). 

I am not sure how to avoid this though. Maybe you'll have some great ideas to avoid this :-) Well here it is anyway, hope this help.

Feel free to refuse this pull request if you find a better way to fix this issue.
Cheers.