Add test for default attribute values
Add test for source-highlighter attribute
Add test for source-highlighter attribute
adds RubyRuntimeContext class to fix issue #93
Adds JRubyContext class for accessing JRuby from extensions and external libraries. fixes issue #93
fixes issue #96 by using JRubyRuntimeContext instead of attributes to pass JRuby Runtime to extensions
fixes issue #90 by allowing register Ruby and Java extensions from AsciidoctorJ
Add methods readDocumentStructure which read detail structure of
rendered asciidoc document for further processing on java side.

Currently it supports retrieving of id, title, style, role, attributes
and content of each block into list of individual chunks.