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


but this does not make sense. The logic inside the case statements depend on the form type being used, (available options are different for each type), not on the data type (we could have multiple forms mapping to the same data type)