Produce nested parameters in FormTypeParser

This is a continuation of this: https://github.com/nelmio/NelmioApiDocBundle/pull/76

`FormTypeParser` now produces nested parameters, just like `JmsMetadataParser`. Therefore the flattening of the parameters into form-compatible names is now fully delegated to `AbstractFormatter`.

This PR depends on a pending PR: https://github.com/nelmio/NelmioApiDocBundle/pull/413


I'm not asking to remove the logic inside the ``else``. Only the ``else`` statement itself as the ``if`` returns