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


it should probably be named ``enum`` rather than ``choice``