Fix ConverterRegistry.converters()

With my last additions ConverterRegistry.converters() did not work correctly anymore because getReifiedClass() always returns null due to the way the converter is created.
Therefore the converters method now checks if the RubyClass has an ObjectAllocator and if so it returns the associated Converter class.

It's not nice, but I also added some Javadocs to this PR for the Converter interface.
Additionally the initialize() method provided by the ConverterProxy now reflects correctly that Asciidoctor ruby may call the constructor only with one parameter, using an empty hash as the second parameter.