Introduced ConverterProxy to let converter impl only see AsciidoctorJ node impls
fix mapping of AbstractNode boolean methods

- help JRuby locate the underlying Ruby method using method aliasing
Merge branch 'mojavelinux-converters-fixes' into converters
Use AbstractNode in converter interface instead of AbstractBlock
Merge pull request #252 from robertpanzer/converters

Introduced ConverterProxy to let converter impl only see AsciidoctorJ no...