Binding User to Edit Form fixes issues #30 and #31

These changes are just a proposal for solution for the following issues:
- https://github.com/Danielss89/ZfcUserAdmin/issues/30
- https://github.com/Danielss89/ZfcUserAdmin/issues/31

I have not edited further code since I can't tell yet what consequences this will have on
- custom form elements
- `EditUser` Form `setUser` event see https://github.com/Danielss89/ZfcUserAdmin/blob/master/src/ZfcUserAdmin/Form/EditUser.php#L72-76
- `EditUser` Form `populateFromUser()` method see https://github.com/Danielss89/ZfcUserAdmin/blob/master/src/ZfcUserAdmin/Form/EditUser.php#L83-99
and others.

But - similar to binding the `User` to the `CreateUser` Form inside the `createAction` method,
this scenario works with *Doctrine*'s ORM with `OneToOne` and `ManyToMany` relationships.

Any thoughts, @Danielss89 ?