further streamlined the API

introduced concept of byte array converters
right now only wired up to top-level index mapping
allowing the removal of the JSON methods, now at the top level
we default to parsing []byte as JSON, override if thats not
the behavior you want.

future enhancements will allow use of these byte array converters
to control how byte arrays are handled elsewhere in documents
this would allow for handing binary attachments, etc in the future

closes #59