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


see my previous review. This switch is dealing with the form types (available options depend on the form type), not on the data type of the bundle (we could have several form types mapped to the same data type but still having different options)